If you are registering to the same server you don’t need two trunks.
Two different usernames should work for registration.
Why can’t your provider just allow multiple calls to the one account? If the provider only allowing one call per trunk it’s not really a trunk nor designed for phone systems.
You want to consider out SIP Station product. It is completely integrated with FreePBX at a competitive rate. Plus you are supporting the project.
We use Sipgate no problem registering two or more trunks, as Skyking says one of our trunks can handle 4 calls at a time never tried more so dont know.
Sipgate settings took us a while to get right.
Peer Details
username=USER NAME
type=friend
secret=PASSWORD
qualify=yes
nat=yes
insecure=very host=sipgate.co.uk
fromuser=USER NAME
dtmfmode=auto
disallow=all
context=from-trunk
canreinvite=no
authuser=USER NAME
allow=ulaw&alaw