A Kiss of Fire & Ice (Gay Heroes, #2.1)

The Alchemists is a superhero team that's falling apart. Part of the problem? Two of the heroes don't get along. To remedy this, they're locked in a single-bed bunker and told they won't be let out until they bond. Though the bonding they engage in might not be what the team was aiming for, it counts. PYRO x CRYO