Yes single loop field with two units. Usually we use an 18 or 25K BTU w2w geo unit for domestic so as long as your loop isn't to big you should be fine adding the dedicated hot water. If you were to do two 6 ton units then your loop may have to much head pressure to be able to be properly flushed. If you think about it the average electric water heater is 4500 watts X 3.413 BTU's per watt is 15,358 BTU's of the average electric water heater so using a 18K BTU dedicated domestic water unit should be about equivalent or using the 25K BTU unit will do more than enough water even at low EWT's, plus the other benefit of dedicated hot water is that while the house is rejecting heat during cooling the domestic unit will be taking the heat from the loop so they work in combination.