Windows IPC....help

I have been thinking the last few days of a best way to achieve this task:

I have two pieces of software. They are both independent application and are final products.(i cant modify the soure).But they both provide an API listing methods that i might want to use to extend some functionality. What i would like to do is as follows:

If i am in PROGRAM A and i click the mouse i would like to transfer the mouse click to PROGRAM B.Or if i drag the mouse in PROGRAM A i would like to do the same event in PROGRAM B and vice versa. The API specifies how to do the clicks and drags in both applications but the problem is how to initiate those functions......?

I was doing some work and i can identify the window handles and thought that i can send something through ::SendMessage(WM_COPYDATA......? Am i on the right track?

There must be some better way to do this....Call the functions from the API from eachothers set?Would that be possible?

I would apreciate any help

Cheers