So just so I understand it, are you slipping the 8x8 over the 7x7 by some amount, and requiring the "joint" between the two to only work through friction and direct bearing of the one tube on the other?
Next question, will the loading cause the joint to slide open or closed? i.e. will there be tension/compression in these members parallel to their long axis?
Assuming no tension/compression (and I understand you correctly), and assuming enough overlap to develop moment, I would design the 2 pieces as one using the smaller section properties. If you can develop a moment at the joint, then I would fix the connection between the two pieces, or even model it as only one piece. You could model it as two pieces, rigidly connected, with the correct properties for each piece. I don't think it would really matter too much which way you went here.
But I would not count on Staad to accurately model deflection, since the two pieces would need to deflect relative to each other, before they would bear on each other to develop any moment. And while loaded, I think there would be too much friction for any sliding to occur.