Place routers like this :
=##=
|
= + | are conveyors
# is a router.
If you have routers in a row like this, they mathematically throw stuff back and forth which in combo with the conveyor leading away leads to what should be close to the ratio you want. This is why routers in a big block take more time than just conveyor belts.
What is it exactly you want to build? Maybe I have some ideas for how to make it happen.
Incase it is what I think it is, to put 2 different materials into a factory, put routers # and junctions % like this:
>#%#%#%>
>%#%#%#>
with either material possibly coming from both sides simultaneously or just from one side each.
Then just lead conveyors to the factories directly from this chunk of blocks, from which the output can be put on one belt.
For more throughput, the thing can be doubled like this:
>#%#%#%
>#%#%#%
>%#%#%#
>%#%#%#
