I wasn't able to get my CO2 up a lot with a 2l bottle on my 10 gal either... neither with bubbling it into the HOB, nor using a Hagen ladder. (And no, there is almost nil surface agitation).
With a 3l bottle it's a little better, still think that it's not enough for whatever reason. Could it be that the hard water (10 dkH) plays a role in that?
Anyway, to answer your question, yes you can connect the two bottles. An even better solution would be to connect both bottles to a third (bubble counter) bottle, both to clean the yeasty CO2 as well as see which bottle is actually producing and which needs to be changed.
