I'd still let'em fight. If the tunnelers in this magical fantasy setting can move underground, know where to pop up and eat the other models on the board, I'd say they can be aware of each other underground. I wouldn't give a charge bonus, as you can't run while tunneling, and maybe I'd make it one action to get into contact with another tunneler, but I'm sure this rare phenomenon could always be settled by anyone at my table, with some chat, and if it came down to it, a die roll. As long as you spell out what you're both doing before battle commences, I'd say roll with it.
In a tournament, I'd have to say play by the book, and make them surface to fight.
If more tunnelers appear in the future, maybe this can be looked at more closely, and the rules delineated a tad more.