Currently I brew 5-gallon all-grain batches using my propane burner and 7.5-gallon brew pot. I've been considering buying a bigger pot and building a bigger MLT to do 10-gallon batches, but I'm not sure what the best way to go about fermenting that much wort would be.
Is it common to simply split the wort into two 5-gallon primaries, or am I better off investing in a larger primary vessel?
Multiple 5-gallon primaries seems pretty common. A few discussions of it:
A few split-fermenters, and one single-fermenter: http://www.homebrewtalk.com/f13/fermenting-10-gallon-batch-75051/
A few split-fermenters and one single-fermenter: http://forums.morebeer.com/viewtopic.php?f=37&t=32355
Either way is fine: http://www.byo.com/stories/techniques/article/indices/19-brewing-tips/226-big-batch-brewing-techniques
On thing to consider is that a 10-gallon container of liquid weighs ~82 lbs, and is very difficult to move. I haven't gone to 10-gallon batches yet, but when I do, I expect I'll be doing split fermentations.
It's pretty common to split. There are upsides: it allows you to experiment with different yeast temperatures, pitching rates, and dry hop additions.
If you want to go with one big fermenter, then you can use a converted keg, bigger plastic bucket, or drop some cash on a stainless conical.
We use a 55 liter glass carboy (demijohn or dame jeanne). They can be hard to clean if you dry hop or if the wort still has a lot of proteins in suspension when it is being transfered. Look in your local classifieds to get one. They are cheap.
I usually just split - it gives you the additional option of treating the two halves differently (different yeasts, dry hop vs. not, etc.) if you want to play around with a recipe.
A couple of options for fermenting in a single vessel that might not require an additional investment. It is fairly common to ferment in modified kegs, so if you have a keg around, you have the option of fermenting in a 15 gallon keg in a single batch. And somewhat less common, but my standard practice, is that I ferment in my brew kettle (chill in place, pitch yeast and use an O2 bottle and aquarium stone to aerate), also allowing you to ferment the full 10 gallons in one vessel.
