Having both boxes on the same sat feed at the same time will lead to NOTHING but trouble. If you insist on proceeding, you will have to plug/unplug the boxes depending on which one you want to use.
Even when "off" the receivers are driving the line, ad may at any time decide to use it - guide & software downloads for example. It's just going to drive you nuts.
You say you have a "dual LNB" setup. If you actually mean you have a pair of LNBs, that means you've got a SW-21 in the line. Look for it.
If you have a pair of Dual LNBs, then you can add a few cables and another SW-21 to git-r-done.
LNB layout Definitions:
Single: One eye, one output jack.
Dual: One eye, two output jacks.
Twin: Two eyes, two output jacks.
Quad: Two eyes, four output jacks.