When I do split enlarging, I use a 0 or 00 and a 4.5 or 5. There isn't any specific order. If you run one first then the other, no difference as the paper receives the same amount of light (in the end).

The typical rule of thumb is to expose the lower grade for 2/3 of the total time and the high grade 1/3 of the time. However, since grades 4 and up require twice the amount of time to get to the same exposure, you basically expose for the "same time".

Usually, I do: grade 00 for 5-6 seconds and grade 4.5 for 5 seconds.

I also split develop, with LPD and Selectol Soft in a 1:5 ratio. The fast acting LPD develops most of the shadows until completion, but the Selectol SOFT is a surface developer and continues to develop the highlights a bit more.

There really aren't any rules to split developing. Most people consider it a pain in the ass to do and only use it as a method to recover thin negs and overexposed negs.