Are you looking to switch between two different sources for your one input? Then you need a switch. Many, many out there, both remote and manually-actuated, for $notverymuch.
Are you looking to "mix" the video signals, in an overlay-type fashion? Then you need an HDMI switcher, which will have other inputs and outputs as well. Will cost more than not very much.
Or are you in some sort of 4K+ setup where the bandwidth available on one HDMI signal path just doesn't cut it?
They're not like analog audio signals, where you can passively mix them together and have it (sorta) work out.
Chip