You can use your Veritas combination plane like a rebate plane to make the (tenon-like) end grain tongue as there is a spur on this plane.
(there are other ways when one has no spur)
That means you have to do the two sides separately.
That is the way people make the tongue to install a breadboard at the end of a board.
See Paul’s video “breadboard-end cutting board” for example.
But don’t try to use it on end grain the way one use a pair of tongue and groove planes (they are not supposed to be used on end grain).