Or, a golf caddy type person.
Just imagine going out on a photo shoot and with the caddy carrying all the kit. Then when you see a potential shot, you ask the caddy for the 7d2 with the 600 on with the 1.4 converter and he/she suggests that at only 35 feet it would be better without the converter but if you crept behind that tree at 20 feet away, you could get away with the 300 lens on the 7d2 or the 300 with the 1.4 on the 5d3 and if you took the shot from a standing position, the shimmer of the hot ground would not have any effect on the focussing but would if you shot from a prone position. When you have conceded that the caddy is right when he/she says that the 7d2 and the 300 without the converter is ideal and he/she had put the correct lens on the body, the subject has gone!!!