Nicholas,
I'll assume that by French windows you mean the same as French door ?
I'm in the process of refurbishing a set of French doors. Completely rebuilding two sets so that they can be used in a 5-1/4" wide wall (vs. std
4-1/2"). Making all new hardwood frames and thresholds and went looking for seals also. Found the borg (HD) carries some vinyl-clad foam seals (V-fold) that have a tail on them that slides into a 1/8" groove and stays snug. Is it the best - I doubt that but it appears to be what is used on most French door sets I've seen. The sets I'm rebuilding will have a center pillar and I can use the same type of seal on all 3 edges (top and sides). If yours does not have a center jam - then you will need a different type of seal for the edges where the two doors meet.
frame |----------|-----| | | | | | |