You could just add a couple of gang valves to the CO2 line so so you can run two off there (and have each one independly adjusted).
However I wouldnt completely trust these to be airtight and there might be issues with the pressure not being consitant between the two valves (so you might end up with one difussor getting most of the CO2 and another getting none.
Those metal manifolds on ebay don't work with CO2 as they are made for airlines.
The actual manifol valve is made to release air if there is too much pressure build up and therefore the pressure needed for CO2 cannot be built.
The plastic gang valves do work but to use it properly you would need to be able to control how much CO2 is flowing down each line as it will not just pass equally down each line.
This is the ADA speed controller and unfortunately you would need 1 on each line but it acts as a needle valve so you can use it on each line and have different flows on each.
This is a cheaper Non ADA version. I think they have used the ADA picture so not sure how identical to the picture it would be but AE are reliable so I would assume it is a good product.
Sorry I'm not following Andy, that aquaticmagic metal thing says for CO2, I cant see a release value on it? It also has separate needle values at either end for each of the two outputs?
You're right about those ADA things though they would work in the same way.