Headspace during extended secondary

[backsideslash]

So I brewed a double batch of an imperial stout that I plan on aging on bourbon oak cubes for the next nine months. Due to the most trub I have ever seen I only netted about 8 gallons, instead of 10, to be split amongst two 5 gallon carboys.

I know that headspace is bad during secondary because of the risk of oxidation, but my question is if adding CO2 to the secondary every couple weeks (whenever I am transferring a different beer to a keg or secondary) be acceptable or would it still result in oxidation over that nine months?

I don't really want to buy two 4 gallon fermenters just for these beers and I think it's too much space to make up with marbles. The only other option I can think of would be to brew a couple extra gallons of that beer to top the fermenters up, but I would be worried about the additional yeast growth and possible off flavors from the autolysis after they die.

Any input would definitely be appreciated

 

[backsideslash]

Brewing an imperial porter right now, I might just sparge a few gallons extra and see what type of gravity I get and then possibly ferment this wort in a separate carboy and then split between the two 4 gallon batches once the yeast drops

 

[tagz]

If it's done fermenting, I would fill the headspace with CO2 and seal it. I usually use a plastic bag with a stopper over top. It can condition and very little 02 will get in.