Readying actions is something you're allowed to do, but it would get really messy and require forethought to do right. Allowing mixing would be the easiest solution.
Here's an example: There's one enemy. You summon the monsters (A and B), they get to act immediately.
The first (A) delays it's turn until after the second (B), which readies the action to attack the enemy after A moved in place behind of the enemy. Then it's A's turn, who moves into position to attack, gets interrupted by the readied action of B, who moves into position and attacks. Then A gets to continue their action (if capable, i.e. unless the enemy is now already dead). -- Both monsters got flanking bonus, thus the outcome is the same as though you had allowed mixing.
However there's an important difference! Here you can't see the results and change your action depending on the outcome. If monster B does in fact kill the enemy, then monster A's standard action (to attack the enemy) is wasted, if I understand correctly. -- That makes some sense if you imagine it happening in non round based mode. Both monsters would be running towards the enemy at the same time and attacking at the same time.
I'd allow mixing, but deciding the entire sequence before starting it.