In message , chris French
writes

However I think I have heard of stepped sealed units - basically with
one pane larger than another. so the smaller pane goes on the inside -
protruding into the inside of the frame. Alternatively it may be
possible to rout out the rebate to make them deeper.

ISTR a one time regular poster Matthew mark did something like this to
some French windows. This link should get you started:

That must be a few years ago, weren't they called Gaul windows then ?
--
geoff