Just clarifying the use of what I take to be the fut. perf.
The kings are asking the two sets of triplets to fight it out to decide which of the two tribes shall rule - the Romans or the Albani (this is Orberg's cap XLIII adapted from Livy)
Eos rogant reges ut pro sua quisque patria dimicet ferro: 'ibi imperium fore unde victoria fuerit'.
The king asked each of them would they fight with ...