Can FineReader Engine 12 and FineReader Engine 11 be installed on the same server?
Yes, FineReader Engine 12 and FineReader Engine 11 can be installed on the same server. They are two different products using separate licensing services, and no functionality is blocked by each other. The only concern here is productivity and performance. We have a FineReader Engine performance guide that explains expectations on the number of processed pages per day: https://support.abbyy.com/hc/en-us/articles/360016440900-ABBYY-FineReader-Engine-Performance-Guide.
Processing documents requires system resources (CPU/RAM) and in the case of simultaneous usage of FRE12 and FRE11, these resources would split. In that case, processing time may be higher than expected.