In the case of the OPs route mix there are two different ways to divide them between both pilots we usually use. One is the 1-4 and 2-3 way, the other one is 1-2 and 3-4. Simply said, most guys do not like to do both their landings at the same airport, a little mix and match makes it more interesting.
Whoever does the first sector depends, there is no rule about that in my outfit. The only rule is that sectors have to be divided evenly, if a captain is unwilling to do that it is a reason for disciplinary action. Of course which sectors are flown by who is in the end the PICs decision, usually depending on weather, fatigue levels and so on. The only exception to that is of course training, where the trainee usually gets more sectors initially, especially on a new type.