When 3/4 full of water, Wallace's water heater has 3/4*40 = <<3/4*40=30>>30 gallons of water.
Catherine's water heater capacity is 40/2 = <<40/2=20>>20 gallons
When 3/4 full, Catherine's water heater has 3/4*20 = <<3/4*20=15>>15 gallons of water.
Together, when 3/4 full with water, Wallace's and Catherine's water heaters hold 15+30 = 45 gallons of water.
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