You dont have to, its just recommended. If you fit one battery that sits at 12.6v for example then one that sits at more or less v, then they will both equalise to same volts ie one discharges one charges. Not seen as being good for your batteries and will shorten their life. The bad battery will drag down the good just like a bad cell on one battery drags the whole battery down. Split charge is the correct way to go but what you suggest is not dangerous or anything, just very basic.