Connect 2 LG Ultrafine 5K monitors plugged into the same side of late 2016 MacBook Pro 15 inch
What you're seeing is the correct behaviour.

Each 5K monitor actually uses 2 DisplayPort 1.2 streams and stiches them together to form a single image.

You have two DisplayPort streams available per Thunderbolt 3 bus, so plugging in a 5K monitor on the left hand side of the MacBook Pro occupies all the DisplayPort capacity available on that side.

This means that you need to plug one monitor into the left hand side and the other into the right hand side of the MacBook Pro. 

It also means that MacBook Pros with only 2 Thunderbolt 3 ports only supports a single 5K monitor.