You should do it through a RAS/Service combination.
On the first IPO setup a RAS acoount. Add an Incoming Call Route of type Any Data that would lead to the RAS account. Finally, in the System tab change DHCP mode to Dialin.
On the second machine create a service, that dials to the other IPOs number (the one you defined with the Incoming Call Route). Make sure you have no quota, otherwise it will disconnect when the quota has been reached. Add an IP route that points to the address of the first IPO with the proper netmask, put the second IPO address as the gateway and the service name as the destination.
Now define the IP trunks on both IPOs.
Notes about setting up the service:
- The default timeout is 10 seconds. If you use both IPOs in a small community, you will have to have a permanent connection between the two machines.
- The bandwidth setting limits the number of concurrent calls you can make between the two IPOs. If your IP trunks are set to G829 or G823, you won't have a problem having 3-4 at once.