Sadly, the Emu can only read a single Master on the IDE bus apparently.
However, with a careful switch flipping exercise in theory you could do what these PC hackers do using the Cable Select but with a removable drive its pointless.
For those interested:
http://www.stefanv.com/electronics/ide_dual_boot.html
Smart versions of this don't power both drives at once - just the one thats needed - however, flipping drives mid stream would no doubt munge the data - the best approach would be a logic gate that only flipped drives after a power or reset cycle to avoid this problem. For the hardcore Emu Modder :-)
M