I would expect most instances to reach the MAXMEM setting at some point unless the DB is smaller than the memory available and SQL Server will not always give the memory back to the OS quickly enough, causing paging. Being cautious, I would be concerned about expecting two instances to run in half the memory they normally run in and would probably prefer them to run at the smaller setting permanently, with the option to increase it manually if you are doing something memory intensive as a special case.