There typically are 2 documents (though they can be combined). There's a lease which pretty much resembles a typical lease.
There's also an option agreement that specifies the purchase price, the time length, the option credits (if any), and those sorts of details.
Some folks include a third document--an actual sales contract--that's pretty much completely filled out. That helps confirm all the details in the option agreement, but the sales contract is contingent upon the tenant-buyer fulfilling the requirements specified in the option and in the lease.
Technically, from a buyer's standpoint, it's preferable that the option and lease all be in the same document. From the seller's standpoint, it's preferable that the two be separate. But either way can work.
As I said, the actual sales contract can be done at the same time or it can wait.
Hope that helps.