You may give a try going together
Some offices are very strict with the infopass appointment lately, but some others may allow you to go together, even with only one appointment.
The only way to know is candily tell the officer at the door you didn't know you needed individual appointments.
At our local office it works (probably not for a long time), if they are not overworked. They may also allow you in, but make you wait until all appointements are cleared.
The good side is if they allow you to get in, you only need to cancel the other appointment, and you will have all passports stamped at once.
Good Luck!
Rich