I've got U. sandersonii and I forget the other. Right now they are in an east windowsill sitting in water up to about 1 cm below the soil level. Does this seem reasonable?
Sure. As long as they are getting plenty of sun. Keep them wet!
They may be terrestrial, but as B28 says, keep 'em wet. Actually, allow the water to evaporate until almost dry and then flood them. Is it possible for you to move them to a more southern exposure? fortunately, a sandersonnii is one of the easiest and free flowering bladderworts. Mine did just fine on a NW window sill.
I only have one south window in my house...and it's already taken...go figure! ;) I guess my east windows are ever so slightly south east, so it isn't too bad. Thanks!