So you've knit across, bound off, and knit across, with two balls of yarn feeding into each side.
Let's say, for example's sake, you knit across (in stockinete), BO, and knit across.
The nexl row (purled) you would purl across all stitches on that first arm, then over to the next arm (this can all be worked on the same two needles you were first using, only there would be two sections on the working needles for each arm of the U shape). On the second "arm", you would BO 3, then purl to end.
Turn. Knit across first arm (the one you just BO three on). On the next arm, BO 3, knit to end.
Purl all stitches on the next row on both arms, then knit all, etc until desired length.
If someone could double check me, that would be great but that should get you BO 3 at each side of neck (essentially widening the neck opening a little more). The staggered BO rows are to accomodate the BO stitches.
Again, I'd love a double check, but I think that'll sort it out.