except... there is one possible reason... In a lot of kits and ARFS the wing joiner is made to be a little small... This allows you to set the exact amount of dihedral... If the wing joiner is very tight on both sides and the dihedral is correct, then you can join one at a time... If it's loose though, you want to do both together so it can set at the right angle... If you glue one half with it lose, the other half may not join correctly when you get there...