Tandem seating surely has distinct disadvantages but that is definitely outweighed by the considerable advantages compared with side by side. vis - Most pilots tend to have a sherbet or two in the evenings and rely on the pilots breakfast (Caffeine, Nicotine, and Vertical speed) to sort themselves out before they can be considered to be anywhere near normal and sociable. In a side-by-side seat aeroplane the overwhelming smell during a rapid climb (especially in a G-corset) is the continual farting and sometimes loud belching as the upper and lower stomach evacuate. This can often lead to a disagreement when there are two pilots, usually in competition with each other. Where there is only one pilot on board, one tends not to notice it so much. Vomiting is also less endearing in a side-by-side as both crew members are subject to the after effects, especially during aero's. Whereas, the tandem usually has a blast shield between the two, cockpit air conditioning is individually selectable, and its very easy to pretend to laugh on the intercom even if you are feeling just as bad, and hence gain the superior or upper hand.
One quite amazing advantage of the side-by-side however is the ability to lean over and initiate a nylon letdown for the other guy. It has happened once, I believe, in a BAC182 Strikemaster which was on a student checkout. The student was briefed to spin 3 times and recover but didn't and locked his leg into prospin rudder. The instructor, a hairy old Master Pilot, watched the altimeter going down big time and the aeroplane was becoming inverted in the spin so the student was left to walk home with a big handful of nylon. I have heard of other advantages of the side-by- side but they involve having a willing (preferably female)passenger, putting the seat pins back in, and partially undressing. I have never given that much credence to this last advantage but some swear it has happened. Others just swear that it hasn't happened... like me.
Dave M
