well, duotops are rare nowadays or it could be easily illustrated
i had one recently and it was an easy win, my shyvana simply went unstoppable as the result
it's actually pretty simple, you don't even need to consider barons or dragons etc:
there are 3 lines and 5 players - 1 goes mid, 1 goes top, 2 go bot and the last one can either go jungle or go second top
the more people stand near dying minions, the less xp each of them gets, that's leaving aside that a lane has gold income only for one player (that's why support doesn't farm)
let's suppose we have a team where one has 2 guys sharing top and another has 1 guy top and another in the jungle
surely at first it will be hard to be 1 v 2, even if jungler ganks, but if you play safely farming at your turret pretty soon you begin to outscale each of those two guys by your level and by your gold; meantime the jungler can farm both jungles (and has a free dragon, unless their mid and bot try to secure it), jungle has almost as much gold and xp income as your whole lane and he has both of the jungles. pretty soon you and your jungler begin to routinely murder the enemy duo top and they can't really come back from it unless you make a grave mistake. their only chance to win it's to kill you fast and kill repeatedly afterwards and to hope that your jungler didn't stomp them late game with all the gold he gets meantime. their bot or mid if those are p. good can carry them of course, but it would be much harder to them to do it
hence why duotop has a high chance to lose and why jungler is important
p.s. in practice one of those who share a duotop eventually usually tries to farm their jungle after a while, but it is usually too late to them