This short post is a mere reminder that hydraulic machines can have more than one oil tank and that not all tandem gear pumps are internally connected at the suction side.
The first time I came across such a layout was during the diagnostics of a tree planting trailer, equipped with a sophisticated hydraulic system that ensured that all trees were planted vertically regardless of the terrain. It had a pretty advanced electronic module, that controlled two DCVs, which were connected to a combination of jacks that moved the complete planting assembly about two axes. The hydraulic part was powered by a small tandem gear pump and I thought I was lucky because there was a detailed hydraulic schematic to direct my "diagnostic efforts". It clearly showed the two pumps, two DCVs, and a single oil tank.
In the course of the troubleshooting process, I needed to test a priority valve I suspected was malfunctioning. During the test, I connected a needle valve to its outlet to simulate the loaded condition. The tank port of the needle valve had to be connected to the tank, of course, and the easiest solution was to stick a hose into the filling cap. Don't laugh now, the expected flow rates were around 3 liters per minute, so the oil shower risks were minimal, and, as the flow was so low, I actually wanted to see the oil stream to evaluate flow rate. (I am aware that this is a bad and not safe practice. But I did choose to use it.)
I turned the clutch on and concentrated my attention on the pressure gauge as I was raising the pressure. When I looked up a couple of seconds later, I saw the oil pouring out quietly from the completely topped-up oil cap! I rushed to the controls and disengaged the clutch to stop the circus, but all too late - the frame, the valves, hoses, and the ground underneath the tank got all covered with oil! And I hate cleaning-up oil spills!
When I calmed down and looked at the machine with the "seeing" eyes, I saw that it was equipped with another oil tank, and the tandem gear pump had a separating oil seal in the intermediate plate. The two separate circuits were working with two independent oil tanks, and during my test, I used the wrong oil tank to dump the oil... I thanked out loud the good engineer who made the diagram, clearly showing a single oil tank, and then slapped myself for not paying attention.
This happens a lot, I mean people assuming that tandem gear pumps always work from the same oil tank. The reason for it is simple - most of the time they do, and most tandem gear pumps have suction sides internally connected. But then there's that one case when they don't, and if you miss it and, say, supply a "normal" tandem pump instead of a "separated" one, or make a test as I did, you end up transferring hydraulic fluid from one tank to the other, with the consequences varying from disastrous to hilarious...
That case taught me to pay additional attention to all tandem pumps and systems using those pumps, and also to never (completely) trust a hydraulic diagram.
So the tip of today is - whenever you come across a double (triple) gear pump with a suspiciously "fat" intermediate plate, do check if the system has more than one oil tank, and make sure you account for it in your tests.