Hi Dmitry,

You don't say why you want to combine/split these chains, so there might be more direct way to accomplish whatever your ultimate goal is than my answers below, if we knew what your goal was.

You combine chains by using the 'bond' command (or Build Structure tool) to form the C→N bond (or O3'→P bond for nucleic acids) between the chains.  ChimeraX will combine them into a single chain.

Splitting chains is uglier because ChimeraX knows that the residues in a chain form a chain even if you delete connecting bonds or even entire residues.  The recipe would be to open multiple copies of your model.  In the first copy, in the chain you want to split delete everything but the first "segment" you want split off.  In the second copy delete everything but the second segment (and all other chains if there are any).  So on and so forth.  Then use the "combine" command to combine the models into one.