I searched the forums and found a couple threads relating to this but not exactly the same. The other posts were about brewing 1gal in a 5gal carboy.
I want to try blending 2 recipes at secondary to allow distinct flavors to meld together. So I want to brew 2gal of the first recipe (14-21 days in primary) and 3gal of the second recipe (14 days in primary). They will be racked to the same secondary carboy for a total of 5gal.
the problem is I only have 5gal carboys. Will that amt of headspace really matter in primary (ie, max of 3gal headspace for up to 21 day). what if I purge the top of the carboy with CO2 after I add the liquid before airlocking?