I haven't used the Unicolor powder kit personally. (I've tried the Arista liquid kit, but I don't shoot enough C41 for it to be cheaper than my local pro lab.)
For measuring other powdered chems I have a nice +/-1 gram kitchen scale. I assume the Unicolor packets are marked by weight in grams. If so, to split a 2L kit in to two 1L kits, just measure out half the total weight of each part and mix with half the required amount of water. Simple.
Also, I assume you have a way to temper your mixed chems at the recommended 100F during processing? One other suggestion, you may want to mix the blix in a glass container instead of a plastic one since adding acid to water can create heat. Although I've never had a significant problem with heat with most pre-mixed blixes. It's more of an issue if you're mixing up your own bleach with high concentration sulfuric acid such as the stuff I use for processing B&W as reversal.