For 13 x 18 cm sheet film, I use the Jobo 2500 system tank that can take 4 120 size reels. I leave the reels out and just put the centre rod in (has to) with some rubber-bands around it, cut off of a bicycle’s inner tire. These rubber bands serve to protect the back of the sheets if they hit the rod. This tank is large enough to hold 2 sheets standing up, straight, on their shortest side and unbent, and the emulsion side facing out.
I put an empty reel on top to hold back the sheets from floating and traveling around. Just be careful not to agitate vigorously, do not roll the tank, the emulsion might scour against the tank.
The inconvenience is that one needs 3,5 L of liquid to fill the tank just for processing two sheets. This is why I develop in concentrated Refinal (= Agfa’s D76), so it can be reused by replenishing.
All tough, a way to reuse one shot developer, like Pyrocat H-D, is to keep the entire amount of the liquid after processing, then to replenish it by discarding about 25% and filling it up with freshly mixed chemicals, and to use it right away. I only do this once because a diluted developer, like Pyrocat H-D or Rodinal, is oxidising rather fast, then I discard the developer. Two sheets are not to exhaust that 3,5 L of developer.
I never tried it with 20 x 25 cm sheets, but is might work too, I guess.
And the (large) Patersons might do the job too, so, no need to buy a new tank system, but do not forget the centre rod…