If the buyer defaults on your second you can foreclose. You would need to be responsible to make payments on the first.
If he defaults on the first they would would be advised and you can step in and foreclose. In any event you want to make sure you have the financial ability to to take over the buyers 1st loan.
I would suggest the buyers investment in the property is at least the same amount as the 2nd you carry Your Realtor can assist you in this process.