How can I ensure that I am keeping track of when I am using their funds and when I am not? I guess I could always open a second trading account which would keep the funds separate, however, this would be a bit of a pain having to sign in and out of multiple accounts.
I was laughing when I read that you are a successful trader but you still cannot figure out how to ensure whose funds are you using when trading.
Anyways I will tell you how it is done. Suppose you have 10k USD which you borrowed from them and 10k USD of your own. Now your capital is 20k and any trade you make is combined of both of yours money. For example you made a 2k trade and profited $50 then you can't say you used your own funds and pay them nothing, that is not how trading works. You need to them pay 50% fro your profits from each trade as you mentioned you have agreed for 50-50 cut for each individual.
if you really think you are professional you need to work on your basics man otherwise you might be just sort of lucky in trading and false believing that you have mastered it.