The two proposed apartment buildings are five miles apart on Philadelphia's Schuylkill waterfront, yet they could be twins separated at birth. It's not so much that they look alike. Each has a different architect and its own distinct style. But their massing and design strategies are uncannily similar, reflecting the best and worst architectural trends in Philadelphia today.

If you compare the current renderings for Ridge Flats in East Falls and 2402 Market in Center City, what you immediately notice is that both are extremely long, but relatively low, buildings. Think of them as skyscrapers flipped on their sides.

In a city where tall towers are often viewed with skepticism and largely confined to a narrow band of Center City, the scale of these low-slung mid-rises will be welcomed with relief by some. At least these nervy newcomers won't be lording over everyone, casting long shadows on their neighbors, or so the argument goes.

Yet mid-rises come with their own issues. Sure, they are less tall, but they often pack in just as many units as a high-rise, and what they lack in stature, they tend to make up in girth. Ridge Flats and 2402 Market each sprawl across a full city block. End to end, 2402 Market would run on (and on) for 450 feet, between Market and Chestnut Streets, while Ridge Flats (also called 4300 Ridge Avenue) extends 300 feet, between Calumet Street and Heritage Drive. The designs represent the ascent of the bulked-up mid-rise as a skyscraper alternative.

Because these architectural wide-loads will face the Schuylkill waterfront and its popular park and recreation trail, their enormous heft should be setting off alarm bells. Urban planners typically prefer when waterfronts are lined with thin towers, spaced to preserve sight lines to the water. Vancouver and Chicago have mastered this urbane arrangement. The parade of high-rises in those places successfully extends the life of the city to the water's edge without creating a barrier to entry.

The challenge for Ridge Flats and 2402 Market will be to keep their long facades from turning into the Great Wall of Philadelphia. Not just on the river side, but on the side where they interact with their neighbors and the city.

Though the developers and architects are clearly aware of these problems, they only half-succeed at solving them.

Let's take 2402 Market first. The 19-story building is the latest from PMC Property Group, the apartment-rental behemoth that has given us One Water Street, 1900 Arch, and the AAA Mid-Atlantic building. In what is becoming a popular trend, it has proposed an overbuild for the five-story Marketplace Design Center, a handsome, 1915 automobile factory that is the architectural equivalent of a muscle car.

Given the monotony of some of PMC's earlier projects, it's a welcome surprise to see how much finesse its architects, Varenhorst & Gensler's New York office, have put into the Schuylkill facade. They approach the project as a layer cake: The old design center is being renovated for offices. Three additional office floors will top the factory. Eleven stories of apartments, containing 321 units, will crown the arrangement.

The best part of the project happens at the Market Street level, overlooking the Schuylkill, where PMC has proposed building an elevated walkway. Although the extension would stop just east of the CSX rail tracks, it would be wide enough to be lined with restaurants and shops, and PMC plans to cut an arcade into the old building so the restaurants can set up tables for outdoor dining. Such cosmopolitan amenities are long overdue for the Schuylkill trail.

Unfortunately, the result of packing so much stuff onto the site is a humongous, 860,000-square-foot building. (By comparison, One Liberty Place contains 1.2 million square feet of space.) To break down the bulk, the architects designed the riverfront facade with a staggered series of setbacks, cutouts, and terraces. The facade treatment (metal panels for the parliaments, glass for the offices) still looks painfully flat, but the details could yet improve.

More concerning is the massive, undifferentiated wall on the Center City side. If the metal panels turn out to be as lifeless as the ones on other PMC buildings, this 450-foot-wide grid would be an eyesore people will see for blocks.

PMC blames the relentlessness of the project's rear facade on the factory's structural limitations; it can't support any more height. (It's now 264 feet.) If that is true, why not reduce the unit count slightly to create setbacks on the Center City side? Right now, the rear facade isn't architecture, but a diagram representing how much space the building can legally occupy. The developers seem to forget that urban buildings are seen in the round and that the back matters as much as the front.

Ridge Flats, designed by Onion Flats and New York's Morris Adjmi Architects, is an almost-identical illustration of the same issues. Located near the Falls Bridge on Kelly Drive, the six-story building would also bring much-needed amenities to the riverfront, including a cafe and restaurant. With a large retail space planned for the corner of Ridge and Calumet, the 206-unit apartment building promises to give the struggling Ridge Avenue corridor an energy boost. The developer, Grasso Holdings, is also considering a single layer of offices.

Like 2402 Market, Ridge Flats has setbacks on the river side and a big honking wall facing the neighborhood. The difference is that the architects have skillfully detailed the facade, animating it with an irregularly patterned window grid and handsomely sculpted trim around the windows. Like Adjmi's other Philadelphia project, the renovated Snellenburg Building at 34 S. 11th St., Ridge Flats was conceived as a contemporary take on the factory loft. There is integrity in the repetition.

But the six-story wall on Ridge Avenue is a bit much for the diminutive houses nearby, and the design would benefit from a shift in scale. Members of the Civic Design Review board, at its meeting last week, noted the wall's relentlessness, yet, for some reason, waved the project through. It still needs Planning Commission approval and a fistful of zoning variances.

PMC's 2402 Market also was supposed to be reviewed at that meeting, but it was pulled from the agenda, and the developer announced Thursday it was modifying the design. It did not specify what those changes would entail. That means there is still time to improve both these projects.

Improving these designs to make them acceptable won't take much more than imagination. PMC and Grasso Holdings: Tear down these walls.