I'll assume you dial 9+number to access the PSTN.
Site A (no PSTN trunks). Build a route pointing to the IP trunks - DN type is public. Build destination code (not line pool code) for 9. Absorb none. Point to the route mentioned above. Add 9 to your destination digits table under the site config for the IP trunks. Also, don't forget to change your prefix default from 7 (or 10) to 8 (or 11). The system will attempt to send the call after 7 (or 10) digits are dialed otherwise. You will also need to build a new prefix of 91 with a length of 12 for the LD calls. May need to build a 90 if international dialing is required.
Give the IP trunks access to the PSTN trunks by building a remote access package on the "far end". For example, add pool A and pool N to package 1, and assign package 1 to all CO trunks and IP trunks. When a user at the "remote" site makes a call, the digits will come across as 9XXXXXXX, and will follow the destination codes on the system with the PSTN trunks.
Don't forget about 911 on the other system - might want to put a COT or two in there for outbound 911 - adjust your destination codes appropriately.