Should I have my parents sign a Lease to show the bank?

[Angie Shires]

We are about to purchase our second buy and hold. The first one is occupied by my parents right now, and they pay no rent. They are elderly and will be there as long as they are able. In exchange, they help us out in our business and around the house. 

It's my understanding that the banks like to see that you have a lease in place for 2 years before they really trust that you know what you are doing. Should I have my parents sign a lease? Will that help and can I back-date the lease from when they took possession 4 years ago (is that even legal)?