I must be missing something here but wouldn't the initial issue, (ignoring combining vouchers from two accounts), be that you can't book 3 seats with 2 vouchers. Wouldn't the minimum be 4 seats?

If you want to pay for 1 seat but use 2 vouchers to get 2 seats free I am 99% sure that is not possible.

Edit: Apparently you can and get a 50% discount on the avios required, (for a single ticket voucher use), so it isn't a companion redemption as such but a purchase using avios. Live and learn!