I used to have a 2661 - if you don't have the manuals for the 2600X, download them (2 of them) from the Kurzweil web site.
Kurzweil uses different naming convention. Splits and layers are both done in SETUP mode. You will build a setup for a split by first choosing what sound you want for Layer 1, then chosing the key range, and any possible transposition (usually octaves).
Then you set up Layer 2 to use the other part of the keyboard with the other sound. I made splits for such things as bass guitar for below mid C and piano starting at mid C on up - once you get used to doing it, you can do it in a minute or two.
Check in the Setup section of the Musician's Guide.