Oracle 11g and 18c in one machine
Can you run 11g and 18c on the same machine for SE2/EE editions?

yes.

I'd place them in different ORACLE_HOMEs.  BUT, you only need to run 1 Listener.

Can you run 11g and 18c on the same OS if both are XE?

no. (But - 18c XE can run in a VM)

11g XE License limits you to one XE instance per physical machine.


  Any use of the Oracle Database Express Edition is subject to the
  following limitations; 
  
  
  Express Edition is limited to a single instance on any server; 
  Express Edition may be installed on a multiple CPU server, but may only be executed on one processor in any server; 
  Express Edition may only be used to support up to 11GB of user data (not including Express Edition system data); 
  Express Edition may use up to 1 GB RAM of available memory.
  


18c XE License does not have Condition 1 from 11g XE.  As I understand, this was done so that you can run 18c XE inside a VM:


  Oracle Database 18c Express Edition automatically constrains itself to
  the following resource restrictions; 
  
  
  2 CPU threads; 
  2 GB of RAM; and
  12 GB of user data.
  


The code also prevents you from running multiple XE instances on the same host.  ( source )


  4.2 Oracle Database XE Installation and Execution Restrictions
  
  Oracle Database XE restricts itself to only one installation per logical environment. The logical environment can either be a virtual
  host such as a VM or container, or a physical host. If more than one
  Oracle Database XE installation is attempted to be started in such a
  logical environment, an "ORA-00442: Oracle Database Express Edition
  (XE) single instance violation error" is raised and the database will
  not start.


But, certain VM technologies are supported.  That is:  You can run multiple instance of XE as long as each one is in a different Virtual Host.


  Oracle certifies the following virtualization technologies with Oracle
  Database on Windows:
  
  
  Oracle VM Server
  Microsoft Hyper-V