one of the double releases your talking about is like this one.
its called the JAMES GREENE GATOR JAW RELEASE CALIPER
they run 50 to 60 bucks here is a link to one.
http://www.yeoldearcheryshoppe.com/james-green-gator-release-p-2179.html?osCsid=7f96851e3dfd973ab031f1afb7006964