Not so. Any legal muster or placement can add a hoplite to a space already containing a teammate's city.
I don't see that in the rules.
The rules for mustering a Hoplite are pasted from the rulebook, page 11, below.
When you muster a Hoplite:
• You must be able to trace supply from the newly mustered
Hoplite to your own City. Your teammate’s City is not sufficient
for mustering (these are your own citizens going to war).
• Additionally, Hoplites must be mustered in the same or an
adjacent space as another of your own pieces.
I thought own pieces meant my faction and did not include my ally. Since the only time you would have more than one piece in a space is when you are mustering in a City, I assumed you can only muster into your own City and not your Allies City.