Any guesses? 
Do I see red and blue PEX connected on the cold side of the right tank?
Edit: I just realized that the red pipe connection on the incoming cold line is probably the return for the recirculating pump loop. They’re kind of expensive when you don’t insulate the pipe.
Never mind. FIxed.
Dang, I looked at that for five minutes and never saw that. 
Sorry, I really didn’t mean to. I thought it would be OK to answer since it wasn’t strictly thermography related. I think it was a creative way of illustrating it.
I’m deleting my response so others can play too. If you delete the answer from yours we can have a do-over.
I was just stuck on the fact you are not showing the temperature scale.
Doesn’t look like any TPRV drain pipe either, at least on the right side.
Also looks like they just mix use red and blue PEX. Here it is RED is HOT and BLUE is for COLD, they don’t mix them, its the whole purpose is to make it dummy proof…wait where is Sean located again?..O forget the dummy proof comment.
Allright ladies keep guesing.
Thanks for the fix Chuck.
It also had QPEX on all the brass fittings. :-k color coorderantionwas the least of my issues with that one.
WAG…Left side water heater - Top thermostat is defective, causing the element to keep heating the water of the top part, and not cycling with the bottom heating element.
Water heaters in series rather than parallel which is preferred.
Right side tank is not heating water.
No T&P drain lines.
Recirc pump looks active.
Also hard piped without unions.
The right side tank may be turned off when the house is not occupied by many people, it may also have a bad upper element since no heat at all is coming out if its not switched off on purpose. If its switched off on purpose, the recirc pump needs to be turned off also or the return rerouted to the left tank.
The recirc pump should be in the return line so its not handling the hottest water (unit will last longer).
I don’t see any check valves, were they present ?
Looks like one or two TPR extension pipes are missing too.
Looks like the left tank hot pipe is pex up to the tank, many places require it to be kept a foot or two from the tank.
Series is fine, its the most common method in residential and it works fine (I have it).
That would seem to make sense, but the mfr does specify that they should be installed at the hot water discharge. http://media.wattswater.com/IS-IHWRS.pdf. My guess is to help keep hot water out of the cold lines when someone opens a cold faucet???
Also, the pump in the picture is cocked. The Mfr says the motor/pump shaft should be horizontal. Not sure if this little bit actually makes a difference, but it might.
I’ve never came acrossed series hot water tanks (yet) or cirulation pumps. I agree with Bruce, I figured warmer water would be coming out of the right tank. Could there be a slight blockage on the incoming line (from the right tank) on the left tank? Temp looks cooler after the 90.
It looks like the downsteam tank is the only one on/working.
If the water heaters are different in capacity, BTU, or fuel types, etc., series is correct.
If two units are the same, series is better. You will actually have more volume of hot water and the units will wear evenly.
Since water heaters should be changed every 10-12 years (recommended to avoid leaks) having them work evenly probably makes sense.
Water heaters were 2007
House was empty so I had to turn on both electric water heaters.
Yes both T&P missing but that was not the real issue.
The right tank was not working. After the heaters ran for about an hour or so I turned on the circulation pump. The left tank produce hot water and the right did not. Since they are in series water came into the right tank and out/over to the left
The thermal was just for reference. I took my hand and touched the line to make sure both were working.
If the right was workng then I should have had hot water. Even if the bottom elements were burnt out I still would have had some hot water.
Right time and situation for the find.