how to install two different chronicles(for example c4 and c6) clients on one pc?

Install c6 in folder A and c4 in folder B? o__o What's so complicated about that?

Depends on where do you want to install them.

You can install them in 2 different disks so there won't be any problem to overwrite the old files. If you want to install them in the same disk then as Grr just change the folders which you are going to install them. But usually when I install different chronicles, I just rename the folder with the old chronicle for instance "C4 client" from Lineage II to C4 and then I install the latest client (example High Five) and it works.

The problem is if you got one version, installer wants to update it instead of installing second. I made it this way: Install client 1, make a copy, uninstall. So u got 1st (copied) client fully working and installer doesnt detect any other version

It's simple. Install C4 to "Lineage2C4" folder, than if install finished rename "Lineage2C4" to "Lineage2 - C4" and uninstall L2C4 in control panel, than Install Interlude to an other folder and repeat these steps as client as u want.

If you did instal c4 first, just rename it after instalation. After that you can instal interlude or whatever crhonic you want, and you will have both.