Actually, cutting out the joiner isn't necessary at all.

Drill a couple of oversized holes into the existing joiners. The idea is to use a couple of short pieces of almost anything strong as "sub joiners". The new joiners are merely to strongly link the existing one. The existing one will do exactly the job it was designed to do after it's strongly joined at the saw cut.

I've used short carbon fiber tubes as short joiners. They'd work here like gangbusters. Make the holes they fit into oversized enough that those carbon fiber tubes don't have to be forced in. Matter of fact, they ought to sorta rattle around. You don't want them to affect the dihedral with their fit. They're going to wind up fitting perfectly and strongly with the generous use of epoxy.

When you've got the two wing halves mating perfectly, and the two new short joiner holes ready, fill those holes generously. Then press the new short joiners into the holes. Any epoxy that squeezes out can be wiped on the wing ribs. The epoxy will fill the loose fit and when cured, the CF tubes will be SOLIDLY and perfectly "aligned".

The idea is to use epoxy's gap filling to advantage. Once the CF tubes support the center of the cut apart joiner, that joiner will provide whatever strength it was originally going to provide.

If you're worried about the CF tubes getting excess weight inside them, simply plug the ends with balsa plugs or tape the ends with masking tape.