I am brewing 5 gallon batches. I end up with 5.5 gallons after the boil so I split the batch evenly into two(2) Corney kegs fitted with an air lock on each gas in. I ferment in the two kegs and then use CO2 to transfer from the primaries into a single CO2 filled secondary (or simply into the serving keg). What I need to know is should I split the yeast between the two primaries or should I pitch one packet of yeast per primary?
In short: use the same amount of yeast.
You should use the correct amount of yeast for 5 gallons, either split between the two cornies or added to the batch before you split it. Dry yeast packets have (more than) enough cells for a 5 gallon batch. Liquid yeast smack packs generally do not have enough cells for 5 gallons of wort, which is why starters are important.
If you don't have time to make a yeast starter then just split it between the two.
But you should look into making yeast starters. I started out using 1 gallon glass jar and shaking it. You need to make larger started but DME is cheap. If and when you get a stir plate started will be much smaller.
Extra step will pay for its self if you start making oversize starters and splitting them. Saving one for later brew day.