First pump slowing down when the second is turned on?

[JaD]

I've been running two DDCs for pretty much forever.

Until now I've had them connected one next to the other with a double nipple, but I've recently taken down the whole loop for maintenance (had to repair one of the pumps), added a res (wasn't using one other than temporarily to fill the loop through a T-line), and thought I'd move the second pump further up in the loop for convenience. There's still no air in the loop, the res is used as a large transparant tube for aesthetic purposes, as I switched to semirigid rubber tubing.

Loop is as follows:
RES->DDC1->VGA->RAD1->CPU->DDC2->RAD2->RAD3->RES

I noticed that when I turn on the second pump (which is positioned after the CPU block, right before the second rad), the first one goes from 4600 to 4300rpm. Can't tell the RPM on pump 2 because I was out of wire and didn't solder the tach on the replacement board.

Why is that? When I used to run them right next to each other, turning the second pump on would increase the speed of both.