I'm in the same situation as you, just i have only one girl 4 years & she's been requesting a DS for her coming birthday in 2 weeks, i personally struggled about buying it or not but seeing the variety of educational games for all ages i decided to go for it.

I would suggest starting buying one station for both with controlled time & duration limits to play with it (45 to 60 min a day) for each with various educational games for each one.