Tried searching for answers on my own, but I couldn't dig up anything definitive.

I have 2 x $400 electronic travel vouchers (e-MCOs) for Air Canada. I'm planning a trip later this year that will likely cost around $600 or so. What will happen if I attempt to redeem both vouchers online for this trip? Since the total voucher value is $800, will I forfeit the remaining value? Or will the voucher remain usable for whatever's left over?

Thanks!