It depends on what kind of fish you plan on keeping. If it's something like a goldfish, I would recommend only one powerhead for circulation. If it's fish that enjoy a more rapid current, then maybe two, but I still think that could be pushing it. Personally I would just get a canister or hang-on-back filter if you're looking for water movement and filtration. I've never been too fond of the UG filters.

I have to agree about a dislike for UG's. Actually, I don't trust them and consider them deathtraps just waiting to happen, but if you insist on using one, do make it a point to do your water changes from the uplift tubes and do it regularly.

As for powerheads, it also depends on the power of the one you plan to use, but one decent sized PH set in reverse flow, with a sponge on the intake and a refillable cartridge on the other uplift tube for outflow should be sufficient for a 29. I've also seen one set up with a coarse sponge on the outflow uplift and must admit the tank held crystal clear. However, that tank owner also knew to stir up the gravel regularly too, to prevent too much accumulation of debris under the plates.

His "trick" and one that's quite effective, was to loosly stuff a large box filter with only floss, then stir up the substrate and run only the box filter long enough for it to suck up the floating muck and clear the water.

Whatever you do though, DO NOT use a UG with fake plants. The entire purpose of a UG is to draw nutrients down and under live plants and a UG does in fact encourage growth, but with fake plants, there is nothing using it, so it sits there, waiting to become deadly gasses that can suddenly release and create a mass die off.

Also, a Whisper 30 is not large enough for a 29. Upgrade to a filter rated for at the very minimum of 2 X the capacity of the tank. 3X is even better.