Not a problem drawing current from both batteries during startup. The starter will only draw the amperage it needs from the path of least resistance which is typically the starter battery. Many rvs have a switch to close the vsr so the leisure battery can help starter battery if needed.
There are different systems to perform the task of the vsr. They use a non-sensing isolation relay controlled by either a bidirectional isolator relay delay (bird) or an isolator relay delay (ird) that sense voltage from both or single direction, respectively.