I used to work at municipal/industrial wastewater treatment plants which had generator hookups for all our liftstations. The breakers for the line power and the generator power were mounted side by side, with a base mount for a bar/plate sandwiched between the breakers, with the bottoms hooked to common lugs feeding the panel itself. There was only two ways to seperate the operation of the bar and the breakers, 1) Use a torch to physically cut away the bar/plate, 2) Disassemble the panel to remove both breakers to get at the mounting bolts for the bar which was underneath the breakers. There was a metal plate or bar mounted to the unit in such a way that both breakers couldn't be on at the same time. Similar to the plate shown earlier. In order to switch to gen. you had to turn the line breaker off (handle down), then move bar to that side, which then allows you to turn the gen. breaker on. To reverse it, turn the gen. breaker off, move the bar in that direction and then turn the line breaker on.
We also had dual feed for a lot of the plant (480v, 3 ph, 1000amp) with manual or powered switching through a knife switch setup. The knife switch was three position with one line feeding in the top of the box, the other line feeding in the botom of the box and the load coming out from the center. Handle all the way up fed line one to the plant load. Handle in the center position was total disconnect. Handle in the down position fed line two to the plant load. Strong point was that each position was VERY positive to insure that you couldn't somehow get them crossed.