That line indicates the switches are interconnected, mechanically. It ensures you can't have both on at the same time. Turn one of them on and it locks the other out. Reason: The major obstacle to paralleling AC sources is what's called 'phasing'. If you connect them and the phasing is out, the damage can be catastrophic! One large boats (work class) with multiple gensets, before they are connected together the engines/generators are 'synchronised'.... Look up 'phase sync meter'...