stay in the same window and use a tab / slide control, fix the problem (bad design) instead of trying to fix the perceived illness.
Can't, and it's not a bad design. Just a default in this design ;-)
If the design isn't managing the potential for record locking, then it's a bad design.
IMHO popovers have a huge potential for interface solutions that otherwise needed a new window (as long as the solution works within the current TO context).
I have a dual window design for one program and it's pretty awesome! It suits a particular need anyway. I don't have the record locking issue you have though.
A quick fix would be to use a trigger when entering a field to jump to the other window, commit, come back to this window and then enter the field. It will slightly lag the system when entering fields and lag longer when a commit is performed so it is not a very good solution but it will work. Put a "freeze window" script step as the first line so there is no jumping windows that is visible to the user.
A proper fix is some sort of redesign of your layouts or data.