When you get both sleeves done you're going to have 3 tubes, that look like this - oOo - when you hold them together and look down at them. Where they touch is the underarm where you would have bound off sts. They get joined later. What you do next is take the sleeve you're just finishing, BO the sts on the underarm and knit around to the first bound off st. Then place a marker, p1 from the Front (they don't tell you this until a few sts later, but that's what you do) slip a stich, p1, place another marker, and work XX front sts, place marker, p1, slip 1 st, p1, place marker, work the sts of the other sleeve, and repeat, only after you place markers you work on the back stitces to p, slip p. Then you're back at the first sleeve which will be your beginning of round.
sue- knitting heretic