Hi mate !
I 'm not 100% sure wether this is correct but I've done it as follows
10hrs logged as PIC (as PF) in the synthetic part of my logbook and the other
10hrs logged as CO-PILOT (as PNF) again in the synthetic part of my logbook
Both are also logged as MULTI-PILOT TIME. If you have the same LOGBOOK as me which is (Jeppesen - Meets JAR-FCL 1.080 Requirements) then is straightforward.
I hope this helps !