Stiebel Eltron DHC 10-2 Electric Tankless Water Heater, 208/240 Volts

I followed "Creative problem solver's" advice and connected the DHC 6-2 directly to DHC 10-2 so that the cold water main line goes into the 6-2 which warms it and then send the warmed water to the 10-2 which makes the warmed water really hot and sends to the rest of the apartment. The apartment is a 350 sq. ft. studio with faucet in the kitchenette and a faucet and shower in the bathroom.

Connecting these two units this way provides scalding hot water to all the taps very quickly. I had professional plumbers install the waterheaters this way to ensure a perfect installation. My tenant is very happy with his hot water and I'm very happy with the virtually maintenance free Stiebels. I highly recommend.

1st time plumbersurplus.com user. [...] perfectly. Used for shower in small older home, and smaller units also used point to point under sinks. I would recommend this company and manufacter again.

I am using two Stiebel Eltron DHC's in Huntington, WV to heat all the water for my 2 1/2 bath home. The temperature outside is 20F and the hot water at the bath tub turned on all the way is just over 100F. 130F at the kitchen sink while turned all the way on.
If it matters I have city water and a pressure regulator that is set for 50PSI.

The way I set them up is like this:

1 Cold water from the city goes into the DHC 6-2
2 Warm water from the DHC 6-2 into the DHC 10-2
3 Hot water from DHC to the house.

I set the temperature controls back one complete turn on each unit and began testing water temperature.

The manufacturer did not intend for the either unit to be supplied with heated water, but it is working well enough for me.

By the way I tried connecting the units in parallel because I read a review online that is would be better to connect them thus.
It certainly didn't work as well for me that way. (By parallel I mean running a separate supply from the city water directly into each heater and then joining the heated water hoses with a Y valve to the house.)
Using that method of hook-up caused the water flow to drop below the minimum to activate the DHC 10-2 resulting in sudden temperature drops.
I might have had the bets success of all if I could have used a flow reducer in the hose supplying cold water to the DHC 6-2, but I had not way to calculate how flow I needed to reduce or how to control the amount of flow reduction.