Controller applied visual separation.
We do that sort of thing a lot at my airport, I restrict the helo to remain south of rwy27 (or whatever I need) and just call traffic to all of the landing aircraft.
I'm not too sure if that's the right answer. I don't know what kind of operation you are running with the helo's and fixed-wings together, but if you are using a surface (another runway, taxiway, helo pad, grass area, etc) you need this separation per the 7110.65:
http://www.faa.gov/airports_airtraf...cations/ATPubs/ATC/Chp3/atc0308.html#Tn850atc
It is true you can have the helo use visual separation between another landing aircraft (Category I+II), but the helo must land behind the aircraft obviously if using the same landing surface.
Here at GFK, we have numerous helo and fixed wing operations. If your not familar with our set up we have two north/south parallel runways spaced about 3500' apart. Our helo's basically operate inbetween the runways. When they need a hard surface we give them the parallel taxiway spaced 200'-300' from the runway.
Per the 7110.65 paragraph that I supplied above, any Cat. 1 aircraft can be side by side from another helo landing or departing from that parallel taxiway. We give a cautionary traffic call to all fixed wing aircraft that will be 'side-by-side' with the helo. Now, when a Cat II or III departs/lands, they cannot be side-by-side at our airport due to the close proximity of the taxiway and not meeting the minimium separation standards. So we either have to hold the helo on the ground or delay the takeoff/landing of the fixed wing.
If your wondering why section 3-8-1 does not say anything about helicopters directly, it's because they all (with a few execptions of course) are considered Cat I (Lightweight, single-engine, propeller driven) aircraft.
There's some other cavats, but this is basically the jist of it. Hope this helps!