I have built a straight A/B box - http://www.buildyourownguitar.com.au...ead.php?t=2481 pretty straightforward and I put links to the diagram. Will differ as it's an A/B - not A/B/Y - that's the next project. Also has the advantage of requiring an output cable to turn on - so no drain on the batteries .
BTW - keen to see how this turns out - have been thinking of making an AB/Y box to run a stompbox and a bass at the same time - get rid of those pesky drummers totally .