Duplex w/shared water heater woe

[Amy Stewart]

Recently bought a multi family in my home town of Port Elgin, Ontario (duplex house and two cottages). The two apts in the house share a water heater. If anyone turns on hot water on either side while you're in the shower you get a cold blast 😓. Is a second water heater the only solution? Or could I get a super bad *** natural gas one that might handle big volume? Do you need a permit to have one installed? I have a plumber coming this week for other issues and I've asked him for a quote. (I would prefer not to do any work that needs a permit as I recently heard that if a permit is taken out the inspector will insist on making it a legal duplex and I doubt it is). Would appreciate your insights!!