@SelfishPopcorn Hi there. You do not do a multi-national attack. No such thing exists - because each Power attacks in its own turn.
However, two Powers successively attacking a common enemy can be a powerful tactic. For example Germany attacks Russia in G2 and then AH follows up in AH3. That can be advantageous even when the R stack outnumbers both G and AH as individuals, but not their combined strength.
AH leading this co-ordinated attack in AH2 is less likely to be advantageous as the G2 follow-up would be after R’s reply in R2, meaning R may have wiped out AH before G can reinforce.
An attacking reinforcement may not involve an actual attack though. Sticking with the same example, a G2 advance into a R territory leads to a battle, but an AH2 follow-up into the same territory does not do so, unless AH so wishes, provided G is still contesting the territory. Which means that R is then faced with the challenge of any attack it decides to make in that territory being against G & AH’s multinational force. This increases the possibility that G & AH will find a co-ordinated attack beneficial, particularly if their rate of turn by turn reinforcements exceeds R’s.
I hope I have understood your query correctly.