You can try calling/texting each other when the phones are in Canada and USA. I don't think it'll work ... but maybe it will, stranger things have happened. Public Mobile is always prepaid so you can't use and can't/won't be charged for any "roaming" you haven't already purchased.
The US Roaming add-ons are an option but I honestly don't think they're that great a deal if the trip is only for a few days, they have a flat 10 day expiry once activated and they can't be suspended/resumed.
I think the standard "US & Canada Long Distance Add-on, 200 minutes for $8" and "US & Canada & International Long Distance Add-on, 400 minutes for $15" are a better deal. The Add-on minutes will get used whenever Plan minutes aren't available but they never expire and will continue to rollover billing cycles indefinitely. They might even be handy for future calls to USA (and beyond).
Other options are the usual email, facebook, whatsapp, fongo, skype, etc. Maybe not so much of an ordeal for a short duration trip. Alternately, one of you could purchase one of those long distance calling cards (with some kind of 1-800 sort of number) which only charges a few pennies per minute, although they usually have some sort of expiry mechanism once activated so not really any better than an Add-on from Public Mobile.
