RE: phoenix decathlon
If you're starting from scratch (to modify that ARF) there is something that would work great to hold the wings together.
Glue strong root rib doublers to each wing half. Have tabs stick down beneath. Bolt the two together to hold them together in flight. There won't be very much force trying to separate them anyway. And the wing hold down bolts are going to do that anyway AND the two hold-down stubs in the wing LE do that also. So why do it?
If you glue in a cross piece in the cabin opening that had a slot for the two new wing tabs, the slot would provide just a bit extra insurance for holding the two halves tightly together AND the cross piece would provide bracing for the wing on inside loops. Wings that are glued together gain measurable spanwise stiffness from being one piece through the center of the span.
But truth is, that wing joiner aluminum tube is quite robust and fits snugly. My ARF had only that one tube, btw.