Admitting that I've read
far too much military history...
- How did those two guys get promoted to General, if they're incompetent enough to get themselves into a situation where this is serious problem?
- Given the stated goal of attacking the fortified City, the City's field forces (in the valley between the two Generals) need to be polished off first. Whichever General is the plausible leader for that (higher seniority, larger forces, better terrain, whatever) should start attacking them, counting on the other General to quickly notice and launch his own attack.