Can you sub 2 on a lease option contract?

Hi I have these lease option contracts and I was thinking of just saying under the terms of payment they get there x amount of lump sum cash (equity) after 2 years(the term of the lease) and then payments of X amount will follow monthly for the mortgage that he holds and just pay like that, is this ligit? and if so do I have to register this at a title company right away or wait until his mortgage is payed off? Thank you all very much for any help

You should learn how to do subject to contracts. On a lease option you do not get title put in your name. You do not get on the insurance policy. You could say that he or the lender will get payments for 10 years but what he he dies, divorces or gets an IRS lien in that time from. You have very little protection.