I understand that Imperium refers not just military command, but also to political rule as well. What I'm wondering is who was allowed to command armies in the field. I'm assuming the two consuls would, but other offices of the Cursus Honorum hold Imperium. Would a Quaestor or a Praetor be able to command an army in the field? Or was it just the Consuls? And if that is the case, what happened when there were more than two armies to command?
Quaestor did not have imperium, Consuls, Praetors and Aediles did. Aediles resided in Rome so they wouldn't have commanded armies, but Consuls and Praetors (and also proconsuls and propraetors) did. In case both consuls were present on the field, either they would each command their own army or they would command the whole army on alternate days. Praetors would also command armies, but since they were provincial governors for the most part, their military actions would in practice be limited to the province they were in.
In case multiple armies were on the field and needed to be commanded separately, officials were allowed to delegate duties to their discretion, while retaining overall command.