It clearly states in the description of boost that the item cannot be applied twice...

SO what should is do
...
one is +26 the other is +24 .. accu

and the reload on both is +19
?

boost affects all stats.

Boost = 20% on the Growth and +20% on the ability.

I prefer to boost when the sailor hits the last classing, but mathmatically calculated of sailors boosted at lvl 1 or lvl 120, it does not matter. (They are only off by less then 50 ability)

Boost is 22.22% on all stats,(crew growth is unaffected) so both reload and accuracy stat will get boosted. You can't use two boosts on 1 sailor regardless of how it's classed, so no you can't do that.

You can boost at level 1 or level 120, it will make no difference what so ever. So the earlier you boost your sailor, the better, because you will enjoy the boost from start to finish of your grind.

Your gunners seem to be base 9+ and 11+ on accurancy and 11+ on reload. Class them both as reload at lvl 65 and don't worry, at level 120 they will reach the acc and rel cap anyways. Have patience for now.