I'm not sure what you mean by more convenient. You book your ticket on aircanada.com, you pay for your seat selection, and you choose your seats. Alternatively, you book somewhere else and you call in to pay for seat selection. Your status only allows you to select preferred seats where complimentary seat selection is allowed (or after you've paid for it on Tango fares).

You'll be able to get the separate PNR's cross referenced and they'll seat you all together; I've done this numerous times on T+ bookings.