Holeigh, you have great input from James. One other option, if you are planning to wrap your brisket in foil, is to just move it to the oven at whatever temp you want (it's not getting any smoke anyway, so you're just after heat to finish it up), and then you are free to do whatever you need to with the ribs. Of course, once wrapped, there's no reason you couldn't keep it on the WSM as long as you have the room you need.
As far as timing on the pork ribs, what kind of ribs (baby backs or spares), and are you planning to wrap those? Those two things will affect how long you need to plan on cooking the ribs, and, therefore, when you should put them on the WSM (oh, and, when do you want them ready?)
You can definitely make this work, just takes a little planning. One thing that I usually do to plan my cook times is to work backward from when I want to eat. So, if I want to eat at 6p....... give myself a buffer hour, spares take ~5.5/6 hours at 250, I'd put them on at 11:30a and I'd be pretty darned sure they will be ready to eat at 6p.
You can definitely get this cook done!! Let us know if you have more questions, and also how it goes!
Rich