Truthful budget partitioning

Question

Suppose there is a budget $M$ that needs to be partitioned into two projects $A$ and $B$. There are two players, where the first player prefers to allocate $pM$ to A and $(1-p)M$ to $B$ (i.e., it maximize its utility), while the second prefers the allocation $qM$ to A and $(1-q)M$ to $B$. Suppose the preference is a smooth function. Is there a fair mechanism (for instance taking the average of both proposals) that is incentive compatible (or close to one)? Can the revelation principle be applied here?