I rarely use a second keyboard, but when I want full 88 note weighted action, I just midi a Yamaha P-85 to my Tyros4. I tend to use the Tyros4's sounds although the P-85 has a nice piano as well.

It's only when I want to do full piano backed pieces, and nothing but 88 weighted/graded hammer action will suffice...76 semi-weighted just doesn't feel right to my fingers for piano pieces.

Ian