Sometimes it is actually possible to learn something on BirdForum...
The question of adult breeding Forster's Tern versus Common Tern has come up several times before here, and a few months ago, Killian Mullarney posted some very helpful pointers not found in any field guide:
Based on this, the identification of the two terns in the photo becomes clearer, even though light conditions do not seem to be ideal. The left hand bird is definitely a Forster's; note extensive dark grey on central primaries, and white wedge on outers, almost like in a Black-headed Gull. Also, note bulging white loral area above bill base, and white underparts. I am not entirely sure of the right hand bird, but it looks more like a Common Tern to me: dark grey on the primaries is much more confined to the tips, there seem to be extensive grey blotches on belly and flanks, and the white loral area is much thinner.