Hi !
It depend on the product you use. Take Xtol for example, Kodak state that you should use 100 CC stock per 135/36 or equivalent surface. If you use it 1+3, you are with 400 CC for one film. If you go to PMK you need about 500 CC at 1+2+100 to do one film. so you'll be overfilling the tank with 2 films in it....
Of course, this is not a problem with Paterson tanks and hand inversion, you can take a 5 reel tank and put only one reel into it, but when you plan to use a Jobo CPE2 processor with 600 CC maximal capacity (in order not to stress the motor) you are limited to some products or a single film at a time...
Of course Rodinal (6 cc minimal concentrate per film) or HC110 are different beast !