All the points for an award must come out of one account. So for a 3 zone Roomette, you must have 35,000 in one account to book that award, which includes both passengers. If you don't have that many points, you could consider two possibilities.
1) Pay to transfer points from one account to the other.
2) Break up the trip. Meaning have one account spend 20,000 points to go two zones and then have the other account buy a one zone award for 15,000 points to finish the trip. Note that this could mean needing to change rooms if the agent isn't careful during booking and you'll have to have the other tickets scanned at the border city where the zone changes.