HAH!!As is usually the case,
not thinking through the use-case, and as a result
not asking the right question, it's really hard to get to a solution!!
I have
solved this problem!!
So what I was asking (and what I was thinking) was about connecting a single USB device to more than one USB host.
"connecting"I didn't specify (and hadn't really stopped to think about) whether both connections had to be active. In this case, of course, it wasn't a requirement. What I wanted to be able to do was to provide MIDI input to apps running on
either of two hosts without having to screw around with cabling, but didn't need to connect to both simultaneously. Thus what I needed was not some way of
splitting the signal, but of
switching the signal.
So what I was actually in need of was a USB
switch.
Turns out there are literally
dozens of these out there, and relatively speaking, dirt cheap. Considering the host-side as the "in" and the device-side as the "out", there are two-in and one-out, two-in and two-out, two-in and four-out, some powered and some un-powered, some just have a switch button, some have a wired button so the switch can be tucked away with just the button on the desk/wall, I even found one that has a IR remote!! They range from around $10 to as much as $40 depending on features.
I bought this one:
Attachment:
USB Switch.JPG
The cable at the top is coming from my Casio piano, the two cables at the bottom L & R go to my iPad/laptop respectively, and the center bottom cable is going to a 5v USB power supply.
And it works like a charm. I can have both my Nektar 25-key controller
and my Casio full-88-weighted-key piano/keyboard plugged into this thing, and push a button to toggle them going into either my iPad or my laptop.
Ta-daaaah!!