Hi there, it is possible to train more than one Merry man in hand-to-hand combat or archery at once per one mission? I'm asking because there is no mention about training more men at once in the manual.

I know that if I assign more men to the certain workshop, they will produce more stuff at the end, but IDK if this works for combat training, too.

If I remembered correctly if you train with Little John in melee or Robin Hood in archery, other participants gets better quicker. You can also train as much men as indicated at the icon above their head.

Yes you can train as many people as you want.

As Zlaywal mentions it's indeed helpful to train with the 'special characters' as they train a lot faster (twice the points.
Will Scarlet is even better in hand to hand combat. I can recommend to do this as early as you can so you'll profit from this in late game when you can't miss these characters.

Also note that the small side missions won't do anything for producing items and following the training, only the larger missions do (the stats in Sherwood might make this a bit confusing).