There should just be a single IMAGE_INSTALL variable. If the package backends need this split into different multilib components they should be responsible for doing this, not the user.
*** Bug 1534 has been marked as a duplicate of this bug. ***
Fixed by http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=1ef442561d9784209f56e8c283824c1f2de3e2d1
I tested multilib against edison branch plus Dongxiao's patch. IMAGE_INSTALL variable could work for multilib now.