As I wrote before during the briefing with the pilots the captain will share the sectors.Sometimes, very often indeed,neither captain acts as PF only the FO's fly .We fly around 6 to 8 sectors per month so it is easy to acommodate .By asking who hasn't flown as PF on previous flights one will decide who will be PF.
And again at end of the day everybody is happy,and btw I think a 12 hour sector is much better than 5 short sectors a day many days in a row.
Best regards .