There's no reason to merge the universes because the two franchises could easily fit within the same one already. The reason for this is the distance in time and space between the two franchises works just as well as having them in separate universes. Of course, this would limit what crossovers you could do. It wouldn't work if you had Trek characters end up in the Star Wars galaxy. However, if non-force-wielders like Hans Solo, crossed over to the Milky Way in the Trek timeline, it could work even as canon within both franchises. You could even establish the reimagined BSG and possibly even Firefly/Serenity as part of the same universe as Trek and Wars without too much problem using the same method. This could even open up new narrative possibilities like having an expedition to try to unravel the mystery of how and why humans existed in a distant galaxy and distant parts of our galaxy in the distant past.
There is however another legal wrinkle or two as well. One is that E.T. arguably took place in the same universe as Star Wars. Also, a weaker argument could be made for the Indiana Jones franchise also fitting in the same universe as Star Wars.