Mr Brooks / Marshall Serial killer

[Ibn Sina]

Hi everyone!
I was wondering about recriating the famous movie character "Mr. Brooks", including his alter ego Marshall (just the same way they work together in the movie), using Gurps rules.
Maybe, but not exactly, Split personnality would suit well... however it wouldn´t be able to mimic the exact relation between the two "personnalities", once Marshall exerts some sort of influence over Mr. Brooks volition but never takes total control of the action.
Any thoughts on how to make it work?
Thanks in advance to all!