I don't think that it'll happen but 2 things are certain:

1) Wlad wants to be the first to unify titles since Lewis.
2) the brothers want to hold every belts together at some point.

My prediction? they will first try to collect all the belts, once it'll be done, there will be one or 2 more fights for Vitali then he,ll retire to let his brother govern the division alone, only to return to avenge his brother if ever he lose in the meantime.
Personally, I think that it is the most logical and likely scenario for both brothers, it would earn lot of publicity, 2 "historical moments" and a load of green bills