I am wondering what would require a 2 pole AFCI? I can't think of any branch circuit that requires it. Maybe the latest NEC has a few? Nothing mandatory in the CEC. I am not saying there are no good or rational reasons for an AFCI 2 pole branch circuit just nothing in the code requiring one.
I am not aware of how the Canadian codes treat what we call "multi-wire branch circuits,' or, less formally, 'shared neutrals.'
This is the practice of running a three wire cable from the panel to the first junction box, where the two circuits are separated. Thus, they both use the same neutral for the final run back to the panel.
For such a circuit, the GFCI or AFCI breaker must, necessairily, control both of the 'hot' wires; thus the need for a two-pole breaker.
Here the more current issue relates to a design change in the AFCI's themselves .... no point opening that can of worms here!
It's also not unusual for the room air conditioner or baseboard heat to be a 240v circuit.
Yes we do have multi wire branch circuits as you describe but if I was doing it I would not use a 2 pole breaker since the cost is more than double a 1 pole. For GFCI we generally install a GFCI outlet at the first outlet and seldom use a breaker. IE normal 1 pole breaker and GFCI outlet at the first required location. No common trip is required for a 3 wire circuit unless the devices are on the same mounting strap like as for split receptacles. It is possible to run 2 circuits from 2 X 1 pole CB on different buss with 1 neutral and a GFCI outlet at the first receptacle in each branch. Obviously the neutrals cannot share beyond this point or the service guy will slap someone on the back of the head ;-) An AFCI circuit requires breaker protection as the entire branch must be AFCI protected. The only outlets required to be AFCI protected are the bedroom Plugs only. No smoke alarms may be on the circuit and lighting may be on the circuit. The cost of an AFCI 1 pole breaker was around CDN $70.00 and 2 pole around $250.00 even for a long home run it is still much cheaper to run 2 circuits.
Oh, I won't argue the economics one bit. Indeed, that's been at the heart of the AFCI debate here!
Originally, they were to allow AFCI devices, so you could handle things as you did with GFCI protection. When the requirement actually took efect (1999), though, the code was suddenly re-written to proscribe the devices. Later editions have made devices acceptable under certain restricted situations .... but the device makers have not made the devices available. Once bit, twice shy! For the guy in the field, it remains a puzzle how a 'protection' available in the device costs only $10 extra, while it costs at least $35 additional if the protection is placed in the breaker.
There are any number of reasons why MWBC's are used ... what is worth noting is that our most recent edition (2008) surprised us with numerous new restrictions upon their use. The next edition has several proposals to restrict them even further. Much to my surprise, one breaker manufacturer (Square D) is leading the charge, with a published opinion that MWBC's ought to be eliminated altogether. Personally, I cannot understand why Square D is concerned about this design issue.