If you are doing an assignment, your buyer should be cash or hard money only. Conventional lenders will not allow it at all. You can just forget doing an assignment at all, unless your buyer is using Hard Money or Cashola.
If you are doing a double closing, FHA will not work at all, as they require at least 90 if not 180 days title seasoning depending on a few factors.
if you are trying to pull off a double closing, the best bet is to get a cool title company, a transactional funding lender, and a Fannie Mae qualified buyer. FNMA loans have no seasoning requirement.
The cool title company has to be cool because you will probably be putting the property into trust to pull it off legally. They have to produce a title commitment to the end buyer's lender.
Or they may be so cool that they produce a title commitment showing you as the seller to the underwriter, even before you own the property? This is less likely though, as I simply think it's illegal and not worth it for an attorney or title company to be involved in.
Bottom line, use an assignment when you can sell to cash and hard money buyers. Use a double closing when you can attract higher end buyers that have Fannie Mae loan approvals. But dont even bother signing a contract with an FHA buyer for a double closing, because you will be wasting your time.