There are several different ways to connect your two swithces. First would need to know versions of the switch? You can use DS1 to DS1 with DCS software, as you suggested you could a MedPro board with IP trunking (not the most economical) or depending on the versions of your switched you can use QSIG but I would need to know what information/features you want to be use...IE Centralized VM, callerid/ANI info, centralized attendant etc. Anyway there are many options available depends on your budget and what solution you are looking for!