Which buildings benefit from engineers?

Solution 1:

Taken from https://www.reddit.com/r/Xcom/comments/44fwdf/what_facilities_can_staff_engineers/

Advanced Warfare Center - 1 slot - wounded soldiers need 50% less time to heal

Power Relay - 1 (+1 via Upgrade) slots - 5 Avenger Power each

Resistance Comms - 1 (+1 via Upgrade) slots - 2 more resistance contacts possible on first slot, 4 on second slot

Proving Ground - 1 slot - Half time required for PG projects

Psi Lab - 1 slot - Half time for psi training

Defense Matrix - 1 slot - Increases turrets' stats during Base Defense

Workshop provides 1+1(after upgrade) Engineer slots, each Engineer in there provides 2 GREMLINS, which count as an Engineer, so basically you can turn 2 Engineers into 4. An easy way to use a workshop to it's full potential is to place the Comms and Power relay next to it, allowing you to staff both of these completely with two engineers. This would also eliminate the need to place the workshop in a central position (though, this will of course increase your flexibility).

Building two workshops doesn't seem to be possible, I finshed one and don't have the option to build another on the spot next to it.