Yes! When's purchasing tickets you can only use two forms of payment and both West Jet Dollars and the companion fare are considered payment methods. So it depends on how many tickets you need vs. How many WJ $ you have. I have had to split a reservation of 4 seats into 2 to pay the extra $ I was short out of pocket.
When I purchased last year, I didn't have to split anything. Used companion voucher and $250 WJ$ from sign up. Paid the balance on mastercard. You can do a dry run on westjet.com to see what happens.