HDCP was a problem for me, because my second TV was only 1.4 (As I was going through my AV Receiver). I got a device that converts from 2.2 to 1.4 which did the job, you'll also need a splitter. If you main is 4K, and the second TV is 1080p, then both screens will only show 1080p, the only way round this is to use a downscaler, which can be expensive. If your not using an AV Receiver or 4k/hdcp 2.2 TV it shouldn't be a problem.
Splitters
This
one works based on the edid of the TV that is on/connected, so you could use the switch on it, if you wanted 4k.
A cheaper option is this
one
Or you could use a manual switch like this
one this acts a passthrough, but you can only watch one TV at a time, this should resolve any hdcp issues, as it will only see one TV at a time.
I have all of these devices, as my system is fairly complex and I've spent alot of time getting it to work correctly.
HDMI over cat6 is another interesting topic.
I used
these to extend my hdmi 10m, they worked fine until I added the AV Receiver, and also include the ir so the remote can be used.
However, I needed something more reliable, that worked with 4k, and an AV Receiver, so I ended up going with a HDBaseT
option, which stopped the cutting out issues I was getting.
Note, for both of these I was using a straight through cable, and have not tried these through wall plates, as I don't currently have any installed, but from what I have read, wall plates could be an issue for hdmi extenders.