i have been told they do, but the boost you get from the second is so small its not worth it.

when i have borrowed another seaman to try it i cant notice the extra on IJN and UK, i havent tried the other nations with 2 seamen so cant comment on them.

They stack, but with quite severe diminishing returns.
A 14 growth Seaman ( 12 base with boost, no nation bonus) at lvl 120 gives about 5% boost to abilities, a second  such  seaman gives about an additional 2% extra and a 3rd 1%.

ok sweet thanks guys

No. Only one breaks through to the egg.

They do stack but they work somewhat like this:

1st works @ 100% ability, 2nd @ 50%, 3rd @ 25%, 5th @ 12.5%


So basicly 2 seamen with same stats increasing a sailor by 1% seperately, together get 1.5%



3 would get something like 1.75%, 4 = 1.875% ect.

Only 1, 2 or 3 are effective, any more are just dead weight