The double close, which requires that you close the short sale transaction first with cash (hard money) works if you end buyer is using cash, hard money or a Lender without seasoning requirements. Simultaneous close - can only be done with a land trust. A HUD-1 must be sent to the shorted lender for approval and the only way you can supply a legitimate HUD-1 is with a land trust transaction. It is complex and not for the novice.
An REO purchase, while technically can be done with a land trust, won't happen that way because the Bank-Owner will not put the property in a land trust to effectuate your purposes. If you have an end buyer, you will have to use the double close scenario and your end buyer must be getting a conforming loan (usually no seasoning requirements).
With these more exotic closings, I suggest using a knowledgeable real estate attorney who is experienced in these types of transactions.
To pull this off successfully, you will need to have the same escrow company and Title company involved with both transactions. Even with that, a lot of coordination needs to be done. When I have situations like this I make sure the Seller I am representing is able to keep possession of the property for 2 or 3 days after close to allow closing and a move to the new property without hassle. So a double closing is not as important as to when possession is turned over. An experienced agent will make sure appropriate language is in the contracts to allow for this.