This is one of those situations that lend itself to testing, rather than opinion. My suggestions:
1. Spread glue on only one surface, clamp it to another board, then take the glue joint apart and see whether the "dry" surface is fully coated.
2. Glue up two samples, one with glue on one surface and one with glue on two surfaces. Let the glue cure well and then try to break the pieces apart.
3. For those worried about the glue developing a "skin". Put glue on one surface and let the glue sit for your selected time. Then perform the first two tests. Report how long you let the glue sit on one surface.
Report your results here. I've done the first two tests - I never worried about the glue developing a "skin" - and I'm very satisfied that applying glue to only one surface is quite adequate.
Mike
Go into the world and do well. But more importantly, go into the world and do good.