Multiple batches into one fermenter

I am thinking that I would like to barrel age some beer in a bourbon barrel, and I have at least two questions. What are the procedures for sanitizing one of these things? I also have two twenty gallon fermenters, and I was wondering if it is bad at all to put multiple batches of beer in each one, and what effect does it have if I were to say to brew ten gallons and wait until the next day then repeat to fill the fermenter?
Thanks
Dane

I've seen some things here and there about prepping barrels, but you're better off searching that out than listening to what I randomly might remember.

As for the blending of multiple batches, I'm getting the impression that's a pretty common practice in the commercial world. I think you'd probably be fine. Although you might want to pitch more yeast when the second 10 gals goes in.

get the barrel as fresh as you can. (from the distillery or winery)
hopefully they put a bung back in it so it would not dry out.
No problem with multiple brews, our club has done several with as many as 15 participants at 4.5-5 gallons each.
no need to add additional yeast into the barrel.
some microbreweries have to brew multiple times to get their fermenters full.
If the barrel is fresh, you do not want to sanitize it as you want the barrel to impart the flavors from the previous contents.