See:
If you look at the SIP registration you will see Primary and secondary registration details. Put your 1st account as primary, 2nd as secondary. Both will then register.
-->> I have done this. Since the primary was already registered it was easy to figure out the details of the secondary which is only another SIP number and password.
In SIP URI Add 2 URIs, in the 1st set it to use Primary as registration and the 2nd set it to use Secondary.
--> There was one URI there (top one) set the ... secondary (strange). I added another URI but could not fill in the local URI and contact. This was filled in automatically.
I have a feeling that the top URI should be directed to the primary and the bottom (newly added) URI to the secondary registration.
In the URIs set the main SIP account to be e.g. Trunk Group 5, and the 2nd to use TG6.
-->> Sorry: lost here. I only have a 0 and 1 kind of info. What exactly is the Trunk group. I would like the pirmary rigistration to be used for calling out to fixed line numbers (that is how it is configured now with mobile outgoing calls using the ISDN line). No outgoing calls possible with the secondary registration. Then calls coming in on the secondary registration SIP number have to be directed to a huntgroup while the primary end up at an auto attendant (which worked already).
Use TG5 for your outgoing and TG6 for incoming as you need.