Using Applied Energistics?
Then an ME terminal with the recipes inside, (4Hydrogen cells+1Carbon Cell = Methane for example) and putting it on top if the chemical reactor may work.
All depends on your GT version, in some later ones you can configure how the chemical reactor handles multiple inputs, if yours is late enough (2.90h should be fine; ie ultimate 1.0.2/1.1.0) this should be fine.
If not, and I am mistaken, a simple way is to have one ingredient being out in the side via an export bus set to craft mode, and the export bus simple have the other ingredient done via the ME interface.
For example, have hydrogen cells constantly put into the side if the machine, and the interface having a recipe such as (1Carbon Cell=5Methane cells) this will however require a chemical really per item you want to auto react/item combination.
Things that double up in one part could double up though, so sodium sulphate and I dunno, something else with sodium.
Try the first way optimally, and the second way if it doesn't work.
