I can't remember and cannot find via search if having more than 1 teacher per student has any effect on the students rate of progression or the multiple teachers stat increases from training.

For instance, I have Fidel, Barry, and one of the 113 explosives experts training a 0 explosive IMP student. I know the rate goes up fast because he has low skill and that the training rate will decrease as he gets more expl.

Anyone?

I'm sorry to say, you can't train skills at 0 points. You will need to start with the minimum of 1!
Skills at 0 are never going to be raised, as i recall. :-/

I am sad to say that it is indeed possible in the 1.13 version to train mercs in skills with value zero.

The requirement has been externalized in 1.13 like most other stuff,
and the value can be set in file Ja2InstallDir\Data-1.13\Ja2Options.ini
with MIN_REQUIRED_SKILL_TO_BEGIN_TRAINING = x
The value was set to 1 in vanilla but someone decided to make the default = 0 for 1.13,
effectively allowing what i call a mild cheat or an exloit.

The opposite happened with TRIGGER_MASSIVE_COUNTERATTACK_AT_DRASSEN = TRUE
which was set = FALSE in vanilla, it makes the game MUCH harder and has probably offended many players new to 1.13
who were not aware that the option to turn counterattacks off even exists,
and how much it affects the game

Nevertheless i consider JA2 1.13 the best tbs game ever in terms of deapth and replayability.
(2042 kills in Danubia on day 9 4:12pm @insane im ;)

As for the main question from @xcorps:
The 2nd trainer has no impact as log as the 1st one is awake (along with the students;)
and has a higher skill as the 2nd.
Thats visible in the students portraits where the numbers represent the chance to increase the skill.
They dont change if you add more trainers as long as the 1st has the highest skill of all.

You will also notice that the left number goes down when the participants energy goes below as distinct value (80+?)
so its more effective to have your mers sleep e.g four times for 2 hours than one time for 8 h.
The even more important side effect of this exploit is that mercs are always fresh when they are attacked.

Thanks! It's been a few years since I played and I couldn't for the life of me remember how that works.