I'm looking to split a 5-gallon batch of mead into smaller gallon batches to try different additional ingredients. Is there any reason that I couldn't go ahead and pitch my yeast and then split the batches, or is it better to split and then pitch?
I suggest splitting then pitching. If you pitch then split you will need to continuously stir when siphoning into the two buckets in order to make sure that you don't get more yeast in one of the buckets. If you split then pitch I think you can more easily pitch an even amount into each bucket.
I'd start off in one batch, once it starts fermenting split it up then. The CO2 will help drive any O2 out of the smaller bottles. Just my opinion, I've never tried this before.
I would split then pitch. Easier to make sure sufficient amounts of yeast make it to all fermenters. Mead is cranky and won't ferment well without lots of yeasts so if any fermenter ends up underpitched you might not get fermentation or you might get some off flavors in it.