Deanna,
I think what you are describing is a "pass through" transaction. If I understand correctly, you are asking on transaction #1 (Frontside) can you simply fund it with the money of transaction #2 (Backside)? From my understanding using the funds from the Backside transaction to fund the Frontside transaction is still legal although not considered commonplace and most title companies wont do it. Additionally there needs to be a document signed by all parties in both transactions acknowledging where the actual funds are coming from. This type of transaction also sets you up for legal and accounting issues that other methods may not.
It's much easier to assign your contract to the end buyer and let their money fund a single transaction (i.e. not a double close). If you are worried about you end buyer seeing how much you made on your assignment fee you can author your assignment in such a way that it doesn't state the amount and you may request a "blind HUD" from the title company.
Lastly, if you are dead-set on double closing your deals, you will either have to fund them yourself and wait on the backside buyer's funds or use a transactional funder. So far my experience with transactional funders has been terrible. Often the back out deals at the last minute, have high fees and points, and their money seems to never be at the title company on time.
If your are just starting out, I suggest the assignment of contract method, it's the simplest, most straight forward, and doesn't require you to spend anything buy your earnest and option monies.
