Your telecom provider is correct, you can't program 2 SIP trunks with the same ITSP details.
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.
In SIP URI Add 2 URIs, in the 1st set it to use Primary as registration and the 2nd set it to use Secondary.
In the URIs set the main SIP account to be e.g. Trunk Group 5, and the 2nd to use TG6.
Use TG5 for your outgoing and TG6 for incoming as you need.
Another alternative is to ask your provider to send the calls to SIP account 2 to the SIP account 1, therefore only 1 account needed. You will get a problem though if you need to route this SIP#2 differently than SIP#1, unless the provider sends in the DDI number/Account Name, and you can match the URI, but this is something your telecom provider should be able to configure.
Good luck!