First,The twos must be connected to power outlets and electrical wiring which belong to the same electrical system.
Connect thedirectly to the wall socket instead of a powerstrip/surge protector. Filtering circuits in the surge protector can interfere with power line technology.
The second question:
200Mbps is the theoretical physical rate. Actual throughput will vary. Network conditions and environmental factors, including volume of network traffic, and network overhead affects actual throughput rate.
Specific factors which may reduce your effective speeds are power fluctuations EM interference from induction motors such as refrigerators, washer/dryer, microwaves, hair dryers, etc.and poor grounding.