Using containment might help you (myNewForm would contain and instance variable of type myFirstForm and an instance variable of type mySecondForm with get and set methods to access the contained forms). Rather than containing objects that both extend ActionForm, it might be cleaner to contain objects that are simple Java Beans. It might be easier to just cut and paste the needed properties.
No, you can't do this. Java doesn't allow multiple inheritance.
Don't over-think ActionForm beans. They're just simple JavaBeans and they're not part of the model. The only important thing is that they match what's on the page. If I were you, I'd just cut and paste the properties I need from either of the two form beans into your new form bean.
While I'd normally frown on "design by cut and paste", in this case I think it's the best solution.