Quad 40mm ITBs might work pretty well on a half-bridge. There are fuel flow calculators you can use, I'll try to dig one up for you.
Edit: There's one at the bottom of this page:
http://www.rceng.com/technical.aspx You can set the number of injectors to 2 to find out the total fuel flow requirements for each rotor, and then you can mix and match injectors to reach this number.
Remember, when the time comes, vids are mandatory.