I know how to share laptop monitor to second monitor if second monitor dosn't belong to another laptop. But in a trip I often have 2 laptops. So how to connect them to work comfortable with 2 monitors as at home. Straight connection with hdmi not working. One solution Program that using net allow use one keyboard, mouse and clipboard on several laptops Who knows another solutions?
You can't connect HDMI between them because that's not possible - the display on a laptop will only accept an input signal from that laptop unless you physically alter the internals of it. The ports on the laptops are for video output, not input.
The only way to do this, therefore, is in software, and there's no simple native way of handling that. There is some third-party software that will emulate this kind of functionality, and it really depends on how you need to use it.
A: If simply using a single keyboard and mouse would be sufficient, and you don't need to actually move windows between the systems or access files from one on both, etc, you can use something like Synergy (which you mentioned) or Multiplicity as a kind of virtual KVM.
B: If you actually need the screen from one to override the other, then you need something more specialized. The only tool I've ever found that seems to work more than halfassedly is Maxivista (it's limited in how many secondary screens it will do but if you're only talking about the two laptops it'll probably work fine).
Note that pretty much any solution is going to require the two to be on the same LAN - an ad-hoc wireless network would probably do the trick in a pinch.