There is no rule covering this issue; however, the common practice is as you state, 60/40 split. The lead driver will also usually get 5-10 percent more than the co-driver. The lead will be responsib;e for ensuring, truck maintenance, fuel payments, and al paperwork are in order. Some folks provide a bonus to the drivers if they achieve certain goals, like time available in a given time period or meeting a minimum monetary level each month/quarter.
However you choose to do it, having your program reduced to a written document is a most important matter.