Either way is correct.
The first form evaluates both (b->ttype != TYPE_void) and b->tkey, both
of which result in either 0 or 1.  The single & does a bit-wise AND on
those two values and results in 1 if both sides evaluated to 1.
In the second form, if first part evaluates to 0, b->tkey is not
evaluated, and if the first part evaluates to 1, the second part is also
evaluated.  The restult, again, is only 1 if both sides evaluate to 1.

So the difference is whether or not b->tkey is evaluated, and evaluating
that is cheap (it's a bool value).