Only solution right now is to calculate the precise number of items you consume, then set up a system with splitters / mergers and different speed of conveyors to achieve the exact mix you need.
For example if you need 60 plates and 120 screws per minute you can use a merger with a mk1 plate input, a mk2 screws imput, and a mk3-mk5 output.
Using splitters, mergers and under/overclocking you can achieve almost any input rate. Dividing the production of your machines and the basic speed of your conveyors by powers of 2 and 3 is rather easy with splitters. For example if you use a fully connected splitter and mk1 input, you'll achieve a 20 item per minute output rate. Dividing by any other value is impossible with splitters so you have to use under/overclocking.
It's sometimes heavy and complex to use such solutions for getting the perfect mix on your conveyor, so a "smart merger" could indeed be useful. However I'd rather have people discussing about how to best implement such a possibility rather than jumping to the "smart merger" solution. We could implement that option on mergers, but also on storage boxes (smart storages), or use conveyors with adjustable speed instead for precisely chosing input rates in front of a classic merger.
I'd rather have the adjustable speed option chosen, as it would allow me to perfectly synchronize input in front of some machines.