Using iMac as monitor
It seems older iMac (mid 2014 and before) can be used as monitors with thunderbolt cable but is there any option or alternative for something a bit later, i.e. mid 2015 iMAc ?
You don't mention what the second Mac is but no, a mid 2015 iMac does not support Target Display Mode.
On the other hand Luna Display or Duet Display may work.