What if a party of adventurers has two magicians or rangers, and each one has Destiny of Heroes? Can they effectively increase the party’s might by two? That is, if they both successfully cast.
Taking the spell description as written, it looks that way. I would think that would be the exception, not the rule. Two spell casters with knowledge of the same spell, with that spell memorized and the need/desire to cast the at the same time? Wouldn’t think that would happen too often.
One of the magicians teaches the other the spell. They both memorize it or cast it from scrolls before a big conflict.
I understand it can be done with the proper planning. I guess i should have said that it would be rare to happen by chance.
As written, yes you can do it. Personally, I would rule that you can only benefit from one Destiny of Heroes spell at a time. Same with Evocation of the Lords of Battle.
I wouldn’t contradict Thor, but I don’t feel it to be specially broken, I mean, the real benefit you can get out of increasing Might is overcoming the asymmetrical conflicts (+2s is an awesome benefit, but in cases of fighting against groups of little creeps a Wizard Aegis or a good Balefire I think would be better). If two point of Might is what separates the party from [maybe] killing a dragon I would permit it.
Stay cool 
I feel the same way. But I wouldn’t want the group abusing the spell, either., just memorizing and casting them all the time just for the advantage. If they tried, I would make sure that there were serious twists/conditions for failure. Or, (as we suffered through with Chains of Fate), put them up against opponents with equally powerful magic.
Or give them lots of moments where they slap their forehead and say “Oh! If only I’d memorized that other spell!”
Also a good idea. Probably a better idea, actually. More of a teaching moment than a punishment.
If you’re going to do something to punish users for using a rule, house rule it so that it’s not an option. Or if you want to allow it but don’t want them to abuse it, talk to them. Ideally you’re playing with smart, friendly and reasonable people… you know, your friends. If you say “Hey, if you want to break this out to stand toe to toe with a dragon, go for it, but don’t use this combo for smashing goblins because it will make the game really boring really fast”, chances are they’ll say “Oh yeah, you’re right!” And if they don’t, that’s a good chance to say “You know, let’s just house rule it so it doesn’t stack” followed in your head by “because you guys are all that is un-fun.”
…or just have them run into a party of shamanic goblins who all know that spell…
I’d allow a double increase if the party managed to benefit from both Evocation and Destiny; having both the Lords and Fate on your side should be powerful…
Totally ignorable post:
Torchbearer is really hard on the characters. If the players find a rule to “abuse”, sure, let them have it!
Stay cool