I know late to this one. But yes, the sulphites stabilise the brew. These stop the yeast becoming active again (often in conjunction with potassium sorbate or similar) and potentially making a brew bomb by secondary fermentation. In addition, they supress other nasties which may cause the brew to go off.
I've made several 5 gallons of wine split into 2 2.5gal fastferment units. Everything splits 50/50.
Just beware of you have the 10l /2.5gal fastferment that the locking ring for the ball joint easily becomes loose when removing the glass sediment catcher. This will ruin your day... you will most likely loose 2.5gals all over the floor! Very poor design!