A submersible pump which pushes it's output from the bottom of the hole is more efficient than one located at the top sucking material up. Most home deep-well water systems use submersibles. For use in a SAGD system one would have to make sure the pump is not susceptable to damage from sand, etc. Any repairs would require pulling up the whole string (Cable and power feed with the pump on the end) to make repairs. This is probably why they are going to try it on one well pair.