I would obviously prefer the starter to rotate out for the sub in garbage time so neither starter gets 40 minutes in a meaningless game.
I've never seen or heard of this happening. The real trick with game shape is to have a balance between starters and backups, or to win blowouts.
i think he meant a situation like this:
A - C
B - C
In garbage it might look like
C
B
all the time, even when i experienced changes there in most time and don't have trouble with a double reserve D there and my trainee minutes. D becomes just trouble, when it look like that:
A - C
B - C -D
when now C replace B in Garbage, D is handled like he have no assigment and comes in for any player without backup.
About the intitial question, 8 player might be enugh but are really borderline, i have some combinations where the double backup choosed alway one position first to replace and with few fouls etc. it leads for to many minutes of the other player. extra scrubs for the double backups, and a team where you can expect some blowuot really helps in the 8 man setting.