Disposals are on for such a short amount of time, they're unliky to ever trip a breaker, irregardless of whatever else is on the circuit- you could already have a 22A overload on that 20A circuit and still never trip it with a disposal.
In your case, I certainly don't see a problem sharing a dishwasher and disposal. In my house, I have the microwave sharing the circuit, too- the duty cycles are so short with so little simultaneity, it's just not a problem. (It wasn't completely intentional or I would have used two circuits... I simply forgot the dishwasher until it was too late to add a new circuit!)
