In reply to sj1992:
I think you want || instead of &&.
BYW, I find it easier to move bins a1 and b1 into separate coverpoints, and then cross them directly instead of trying to create complex bin expressions. Especially if bins a1 and b1 are really multiple bins and you want to retain the cross product autobins. When you create an explict cross bin, the autobins get collpased into a single bin.