Let me see if I understand the question. You have a semi transparent object, which you want to move under another semi-transparent object. You want to find how transparent the 1st object will be under the 2nd. right?

Well, if you could convert the two objects transparencies to 0 to 1 (50% would be 0.5) then I would think you could simply multiply the numbers together to get the transparency of the bottom object.
Or is that what you've already tried?