How I do it is very loose. I am solo also (not living with either of my lovers). I have my own schedule (work, school, etc.), and then it's first come, first served for the guys I'm seeing. Neither one of them tends to plan too far in advance, and neither do I unless it's something special happening, like if we need to buy tickets for it. But whoever makes plans with me first gets the date that works for us both, and then if the other wants to see me that day, I just tell him I'm not available and we schedule it for another day.
I wouldn't be able to stand it if my life was scheduled to the nth degree, but since you know that your primary has a 2-week cycle, there is some built-in predictability which does help. Do you have at least one day a week you can see him when he has his kids? Is it something that always changes or is there a pretty regular day he can meet you?
So, if I were you, I would probably designate certain days/nights for him and days/nights for her that would stand no matter what the cycle is (he has his kids or not), and then every two weeks you can let them know you are available more often for extra days with one or the other lover, but those are more loose and spontaneous. Does that make sense? It does to me, LOL. Let me know if I should explain it better.