Again, it depends on the work. If, for instance, 'the work' consisted of stuffing 500 envelopes and there were five people, then you could say, "Let's split the work- 100 each", whereas, if the work consisted of stuffing envelopes, arranging seating, getting supplies, etc., then you might instead say, "Let's split this up. Tom, you are in charge of stuffing the envelopes. Jane, you arrange the seating, and I'll take care of getting supplies."
I'll add that personally, I will most often use split to refer to an equal division into two parts, and split up if there will be three or more divisions or I might say, "Let's split this four ways."
Just to confuse you a little more, when a couple are no longer together, we refer to them as having 'Split Up'... :roll: