Wirelessly posted (BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)

If you are willing to make a double connection, do not overlook flying EI from BOS or JFK to DUB, and thence to BCN. When redeeming Avios for TATL flights on EI, you will pay almost nothing in fuel surcharges. (An award flight from DUB to BCN will have higher surcharges, however, but Ryanair also flies that route.) Avios redemptions on EI require a call to the BAEC Service Centre, and payment of the telephone booking fee. Since BOS-DUB on EI only costs 25,000 Avios per one-way in J, it might be worth your while to see if you can get a cheap paid ticket between TOR and BOS. (Does Porter fly to BOS?)

All redemptions with Avios, regardless of carrier used, are based on a per-flight basis. So it makes no difference in terms of how many Avios will be required whether you book two one-ways or one round-trip.

Good luck.