If DS1's toys are mostly high tech, I'd go mostly low tech for DS2's toys. Lego Duplos and PlayMobil 1.2.3. are specifically for younger kids, and even if DS1 had some, you can always find new ones for DS2. Or try to find add-ons for existing toys, to make them more fun/exciting, and to change the way they play with the toys. For instance, if your DS1 has lots of toy cars, then get something like these for DS2:
And my DD (2 1/2 years old) loves this, and while she isn't using it the way it is intended, she will grow into it:
http://www.amazon.com/MindWare-44204...dp/B0037Z8DEK/ (And something like this, both kids should be able to play with)