I found the following post on voxilla, and thought it addressed what I was trying to do:
http://forum.voxilla.com/asterisk-su...000-11703.html (Trunk group config with two SPA3000)
The only difference I want to do this with SIP trunks, which these esentially are after the PSTN connects to the ATAs
So I placed call-limit=1 in the SIP.conf for each trunk, just to test, and the first call routes successfully, but when I try to place the second call i see something line this
chan_sip.c:2259 update_call_counter: Call to peer 'trunk1' rejected due to usage limit of 1
-- Couldn't call 5XXXXXXXX@trunk1
the second trunk is never used!