My home has 2 wh serviced by 1 inlet with a prv at the point where water enters the home. Each wh services half the home (ie west wh services west side and east wh services east side). Each wh has its own shut off valve. Calif plumbing code 608.2 say an expansion must be installed downstream of the prv. No other requirements except to follow manufacturers instructions. Manufacturer states same with no other requirements. Does that mean I can get by with 1 expansion tank for both wh or do I should I install 1 for each wh.