What you you should have done here is casting on the front and back sts to the sleeve in a T shape. So you didn't need to do them with separate yarn, just cast on at the beg of a WS row for the front, and the beg of a RS row for the back.
But since you've got them like this, just keep going. Slide the back stitches onto the other end of the needle (put the tips together and slide them over as if you'd just knit those sts) so you have some working yarn at the end of the R needle. Then knit the sleeve sts then the front sts and turn. Work back over all the sts to the end of the row. That will join all the pieces.
sue- knitting heretic