I want to split my primary

I brewed a robust porter and has been in primary for 2 days. I would like to make a cherry porter, but I wouldn't mind finding a way to only make half the batch the cherry porter and keeping the other half as a porter only.

My plan was to add cherry puree to my secondary and racking from my primary on top of the puree. I've read many articles stating that I should transfer from primary when fermentation begins showing signs of slowing down. So, I'm thinking about racking half of the primary into the secondary (onto the puree) and leaving the remaining half in the primary to finish fermentation in the primary.

Do I need to be concerned about having only half the beer sitting on the majority of the yeast and the cake? Any concern over having so much head space in the primary? Can anyone think of any issues with doing this?

Thanks!

Ideal would be racking to 2x 3g carboys...
I bought 2 of them so i could do exactly this. Also works well with the 3g corny kegs.

My neighbor has a 5.5 gallon carboy I could borrow to do this - any issues with leaving so much headspace in the carboy (considering it's a 5.5)?

Some will say that much headspace could be a problem for fear of oxidation. Others will say that if you keep the carboy relatively still, then a blanket of CO2 should form over the beer and keep the oxygen above the beer.

I tend to fall in the latter category.

The yeast isn't a problem. After you split, give the primary a good swirl. That will force more CO2 out of the beer and fill the headspace.

Thanks for the advice all!

Just wanted to post an update on this - after racking half my beer onto my fruit puree and leaving the other half in primary, I gave the primary a good swirl....after about an hour I've got some pretty good bubble action in my airlock. Assuming this is CO2, it should be pushing all oxygen out of the primary. So, I'm thinking (and hoping) things went OK.

Sounds like the swirling got the yeast going again. No problem there.

I have, however, noticed that the half I racked onto the puree stopped bubbling in the airlock after a couple of hours. Anything to worry about there? Would a swirl be OK on this one, too?