you shouldn't have any problems with two c-more connected to a single ecom (or ecom100), a reasonable limit is somewhere around 5 or 6 panels per ecom.
the issue is more one of acceptable performance than functionality. the single biggest point of concern is the plc scan time because the ecom only gets one opportunity per scan to process it's comm requests and the plc will only accept one type of request (either a read or a write) at a time.
so you can see there will be a direct correlation between the plc scan time the responsiveness of the data through the ecom to the c-more panels.