there are a few reasons they went left and right hand bowls - particularly for minis, but most SU setups after the 50's were left and right-hand....
1/ it looks better - have a look at that set, they look silly
1.5/ they can charge more for them
2/ easier to tune and balance - you can get your hand down between them, and there is a specific method of setting up twins properly where you set the clearance from the boomerang shaped thingy on the throttle linkage to the choke linkage - the choke linkage aint there
3/ twins were only sold on the performance minis - the ones that were supposed to go racing. With 2 left hand bowls, what happens when you go around a hard left hand corner? both carbs lean out, killing your chance of hard acceleration out of the corner, with a left and right hand, one leans out, the other richens up - compensating, therefore you can still go fast
there are other reasons, but I forget... the 3rd is the one you get from books