The Paterson tank the OP is using is over thirty years old. It is very unlikely to be liquid-tight during inversion agitation. I certainly recommend buying a current Paterson model, with the full-width sandwich-box style lid. The filling and emptying of the solutions is far quicker and the tank is much easier to clean. The reel size is the same. Tank sizes are available from Paterson which will allow two (and more) reels of 120, each holding two rolls of film.
If the tank centre-tube, and/or reels, are so worn that the reels move up and down then this can be prevented by rolling a rubber band down the centre-tube on top of the reel. Or by using the clip which used to come with even older Paterson tanks, before they improved the reel fitting.
The liquid amounts moulded in to the bottom of the tank are adequate. The tank will agitate adequately with two 135 size reels (2x290ml), as it was designed for that amount of chemicals. Note that there may be a very slight difference in the effectiveness of inversion agitation when leaving different amounts of air-space in the tank. This is because inversion agitation actually moves the solution around the tank by shifting the air-space from one end to the other (during the inversion) then the solution flows around the reels and through the centre-column. A wider film, or more reels, means a greater volume of solution, which means less air-space, which means proportionally less agitation effect. If such a difference is actually noticeable, either the number of inversions or the processing time can be increased but the main thing is to make notes and to keep consistent in order to retain control of results.
If one decides to fill the tank to the top, in case the reel moves, it may be helpful to place a four-leaf clover behind each ear and to wear green socks in order to improve the consistency of each development session.
