"So I'll be picking up 40 from the front piece and 40 from the back piece? Your advice is to divide these in half and mark where I'll pick up the halves (20 stitches)? I'm using the circular needles for this as well, right?"
Yes, pick up 40sts on the front and 40 on the back (20 in each section if you divide the front and back in halves) and the circular needle is fine. You'll be knitting back and forth in rows and not joining in the round.
You don't have to pick up very precisely along the armhole. For the front (or the back) you have 10 inches in which to pick up sts. That's about 60rows (you can count how many rows you actually have). If you pick up 40sts in 60rows, that 2sts/3rows. A stitch or two, more or less isn't going to make a big difference. You can always add or subtract a few sts on the next row. The markers are just to make sure you don't get to the end of the armhole having picked up 70sts or 90sts.
Give it a try. Try to pick up along the same column of sts and it'll look beautiful.