Extending sections of HDMI cable with a coupler

Splitting HDMI as a regular Y is not a good idea (because it's a two-way protocol), but there are splitters you can guy.

Extending by another 10' could work, especially if you do 1080i or 720p. If you want to do 1.3 "deep color" stuff at 1080p/60, you might be in trouble. For that case, try putting an equalizer/repeater at the joining location, which will make sure your extension works fine.

HDMI in general doesn't "degrade" the signal -- either it works, and it works well, or it doesn't work, and it'll be obvious. The narrow transition band between the two would appear as "flakiness" or "sparkly pixels" more than an overall degradation in quality as you would find with an analog connection.