Assuming you are not meaning the Assailant, what you are probably referring to is the "Hero's Double".

You get them from packs. They only last a short time.

e.g.

Just remember, everything else is the same. All you are really doing is sending out the Hero twice as fast, essentially. So you are using twice the stamina, running out of stamina twice as fast, and so on.

It saves times, but the game has a hard time with keeping both in motion when attacking an Assailant on the next tile. I continuously get "can't do this action, all heros out" and it blocks, or one ends up staying home because it isn't evident that they're out. [The game wants to notify "Hero sent", "Hero struck", "Hero returned", and the notifications get in the way of each other, particularly with the Double, obscuring that one has has returned and is again available to be sent.]

It may or may not be worth it, when. e.g. If it is two days until event end, probably save it. If it is two hours until event end, it is probably too late, most times, for the Double to secure the win for you. You might get to the next checkpoint, but the Double is an expensive way to do it, and checkpoints almost never give enough back to be worth the cost of getting it this way.

That same principle applies when used otherwise. Be it yielding a 2nd tile at Hero speed, or attacking a town twice at a time - if you have enough higher tier troops to make it worthwhile, and your victim doesn't shield or move in the mean time.

It can also be used to send a Hero out on attack, while leaving one at home to defend.

Just remember, the Double is time limited.