Merkle tree H(RBIT(X) ⊕ Y) instead of H(X || Y) I was reading Can XOR be used in a Merkle Tree instead of concatenation? and had the idea of reversing bits for either X or Y before the XOR combiner, to indicate the order. So instead of H(X ⊕ Y) we could use H(RBIT(X) ⊕ Y) (or H(X ⊕ RBIT(Y))).

The cost of both RBIT and XOR instructions are negligible on most modern CPUs compared to an additional hashing round, performance would (almost) double...

Assuming also that X and Y are mutually independent (which is the case in many constructions where the leafs of the merkle tree are pseudo-randomly generated, or randomized), would this make a secure combiner? Why (not)?
The problems mentioned in Can XOR be used in a Merkle Tree instead of concatenation? are just special cases of a fundamental problem: the construction is not second-preimage resistant. In fact, it is even worse in the sense that you can find a preimage with any $X$ value you like (similarly any $Y$ value you like).

You can open a Merkle-tree node $H( X \oplus Y)$ to any other values $X'$ and $Y'$ as long as $X \oplus Y = X' \oplus Y'$. If you can do this, then you can open a Merkle-tree leaf to literally any value. Flipping/inverting some bits (and indeed, any linear operation) won't make this attack any harder.