Hi,
You need some external logic like a couple of AND gates. But how to do it depends on if you're doing locked anti-phase PWM or if plan to use one switch in each leg for the PWM and the other as "steering"?

With that said there are small PICs a lot more suitable for this kind of application, like the 16F684 for example, which has an enhanced PWM module more sutiable for driving a full H-bridge.

/Henrik.