Okay...so, to give your heroes this ability:
In my game system, characters use gold and treasure to train to learn magic, combat moves, knowledge, etc.
So, it doesn't really translate to HQ very well, in that sense.
However, each of the cooperative casting effects...flame funnel, electrical cloak, and mudman wall...cost
300 gold for the training. One hero would have to know one spell, and the other hero would have to know the
other spell, and the training for teaching them to use those spells together is 300 gold...I suppose 150 each, if
split between them.
If you allow your heroes to gain some sort of skill card or similar "leveling up" method, I would say make them
use two different leveling options. Maybe have a training card for the first card, then the actual ability for
the next card they can choose. It should not be a very easy thing to learn.
(Realize, in my system, they would have had to train to use magic, and train to learn the base spell, and then
train to learn the cooperative casting ability for each effect...and there is a time element included, as well.
Each effect takes 8 hours per day of training, for 15 days in a row.)
If you want to use a gold amount for learning this ability, I would keep it at 300 gold. My system is somewhat
different from the basic HQ in terms of how much things cost and the BP and MP and spells and such for monsters,
but not so much that gold amount cannot transfer directly. At the worst, if you want to change it...I would not
make it cost less than 200 gold.