Assuming user control A has a reference to User Control B...
Assuming the methods yo u want to access are public.
From within A you can simply do
instanceOfB.SomeMethod();
If you want to access methods of A from B in the same situationm, you will have to give B a reference to A.
One way to do this would be, in A
B.InstanceOfA = this;
where InstanceOfA is a property in B
Now B can call methods of A using
InstanceOfA.SomeMethod();
There are better ways of handling this but they all depend on what sort of project you're dealing with - and you haven't given us a lot to go on!