It is a chancy proposition at best in my opinion, very unlikely to work unless you get lucky and get exceptionally tolerant individuals of both species.
Even then neither would appreciate the presence of the other, since as mentioned Labrynth fish tend to be especially territorial to in their own family, and Betta splendens being one of the more territorial species in the group.
Edit: it could also appear to work at first, only for one or the other to without warning decide to declare war.
For the above reasons, I am adding my recommendation against this arrangement, and in full agreement with your decision to keep them separate.