I am looking to book one way from SEA to Canada with stopover in Europe, so I am trying the multicity tool SIN(or BKK) to SOF and then SOF to YUL on the AP website , but won't find anything.

If I search them as separate one-ways, I get tons of flights on both (e.g. SIN-IST-SOF and then SOF-IST-YUL).

Would I be able to book over the phone as one ticket (at lower mileage against 2 one-ways), or they will try to tell me that this is not a valid multi-city?

Thanks