From a purely-technical perspective, I don't believe account-to-account direct transfer is possible in an ACH file. The file format (see part 4 of that article) of the "PPD Entry Detail Record" specifies one end of the transaction, but the other end is always the entity transmitting the ACH.

The other big challenge: what happens if the ACH gets reversed? There's a time lag between when the ODFI pays out to the originating entity, and when the ACH is irreversible.