As it was explained to me, the problem with a true dual shelf bow (in my case thinking in terms of straight longbows) is that you significantly weaken the riser/bow in terms of sideways impacts... you're going through the woods and bang a tree and the bow may break in the middle, where the riser is thin, from sideways pressure.
Generally, the shelves are not cut as deep, i.e. toward center, to try to keep as much strength in that area as possible. On bows with thicker risers, it is obviously not as much of a potential problem, but you are still going to end up using a lighter spined arrow as you're not as close to center.
I build ambidextrous bows, but they aren't everyone's cup of tea, as my solution is pretty simple... I don't cut shelves into them at all, just install thin leather "rests" that are really just arrow placement references. You're actually shooting off your hand, as on the old English longbows. Some folks enjoy that, it takes them back to the kind of shooting we did as kids. Others don't like it. Arrow matching is a bit more critical, you end up using a lighter spined arrow because you're shooting around a thicker riser section.
I built one like this as a traveling bow for the Howard Hill Longbowmen and everyone who has used it liked it a lot. Here are some pix:
Thin leather wedge inserted in wrap provides arrow placement reference point and also a "feel" reference for the grip. The leather dot insert provides a soft surface for the arrow, just as a shelf pad does.
Same thing on both sides...
View from the front.