In short, you can't. Maya won't collide two soft bodies. I tried faking it once by running a script taht creates a particle for each cv of an object, and constraining the cvs to the particles, then adding springs. So, the base shape was actually a rigid body, and therefore detects collision with the softbody. However, I did not get very far with it and found it to just be too buggy to be worth developing. The main problem, once it was working, was that although the soft body deformed as it should with dynamic forces or surrounding objects, and the faked soft body object deformed around the real soft body, the real soft body did not deform around the fake one. And so, they weren't really interacting. Anyways, its an extremely flawed theory, dunno why I bothered mentioning it really.
Anyways, there are other solutions. You could bake out a soft body colliding with a rigid, and then turn the baked geom into a rigid, and the other rigid body into a soft body, and so on and so forth. Not particularly convincing, but can be a reasonable cheap work-around.
Another solution is to use two cloth objects. That can work well after spending a lot of time getting it to work.
The other more sensible apporach, is to use something like Real Flow....it would do not just lava-lamp type stuff, but would also do more solid things like rubber objects or fluid with a lot of surface tension.
