They kind of are though -- if Lena is working for Seymour she's prohibited from doing other work, so she just tags along with Ivy as "vacation trip". Perhaps there's even going to be some choice for Lena at Billy's place to abide by that rule, or break it, with potential consequences. I'd imagine drug use might play into that, too.Seymour and Billy could have easilly been mutually exclusive in the same way Ian pursuing Cindy or Holly is, instead of just being left openly available.