GTX I saw the twin engined A-4/DC9 hybrid a few posts before, and I'm not sure that'd work as getting a pair of engines powerful and small enough to pod outside the main fuse would be an issue not to mention the severe aft CofG shift it would cause.
It might work with something such as a Honeywell/ITEC F124 or Rolls-Royce Turbomeca Adour though getting the right balance of weight, thrust, full consumption etc would make it difficult to argue for. Only reasons I could see (outside of the "it just looks cool" reason
) would be if one specifically needed two engines for survivability reasons or even as a dedicated two engined trainer or, as you mentioned, one had plans for the leftover internal space for more fuel or other.
Taking some of this, one could potentially have a scenario such as this based around
a different A-4AR Fightinghawk:
Argentina had a long history with the A-4 Skyhawk and in fact was the first foreign user of the Skyhawk in 1965. Over the years the country suffered many challenges in supporting these aircraft ranging from embargoes through to losses during the Falklands War. These along with a less than stellar national economic situation denied the resources needed to replace the venerable Skyhawk. That said, the crews still worshipped the Skyhawk for its rugged simplicity and ability to punch above its weight time after time.
In 1989, Carlos Menem was elected President of Argentina and quickly established a pro-United States foreign policy. Although the economic situation improved, the funds to purchase new combat aircraft remained unavailable.
Eventually in 1994, the United States agreed to a request to purchase 36 former US Marine Corps A-4M Skyhawks in a US$282 million deal that would be carried out by Lockheed Martin and included the privatization of the Fabrica Militar de Aviones (Military Aircraft Factory), which was renamed Lockheed Martin Aircraft Argentina SA afterward. Fuerza Aérea Argentina technicians chose 32 A-4M and 4 TA-4F airframes from the Aerospace Maintenance and Regeneration Center at Davis-Monthan Air Force Base in Tucson, Arizona to upgrade. Originally it was planned to simply overhaul of the airframe, wiring looms and the Pratt & Whitney J52P-408A engines however once the aircraft arrived in Argentina it was discovered that much more was needed. Most importantly, not long after acquiring them Argentina like many others around the world were hit by a Pratt&Whitney announcement that the J52/JT8 had a major fatigue flaw requiring its immediate removal from service (P&W frustrated many in that it also decided to not continue production of the engine components thus preventing a simple change out of the affected items).
At first Argentina, like many others looked to copying the Singaporean Air Force and replacing the J52s with non-afterburning General Electric F404 turbofan engines. However despite numerous requests including high level political delegations, the sale of the F404 to Argentina was denied. For a long while, it looked like the Skyhawks' days were over. Then there was a break through. Argentine engineers in conjunction with their counterparts half a word away in Taiwan (another country that struggled with on-again/off-again US support for arms purchases) came up with an innovative proposal. The A-4Ms would undergo a major redevelopment utilising many of the items from the Taiwanese AIDC F-CK-1 Ching-kuo. This included the F124 engine (the AIDC actually using an afterburning version of this designated the F125). to get the necessary thrust, two F124s were to be used on the A-4. These would be mounted in external pods on the rear fuselage. Although somewhat ungainly looking, the resulting installation freed up much internal space in the fuselage allowing an increase of fuel load by 85%.
The resulting A-4AR Fightinghawk as it was soon designated also introduced a new radar (the same GD-53 Golden Dragon multi-mode monopulse pulse-Doppler radar as used in the F-CK-1) along with other new avionics and related from the Taiwanese. Eventually entering service in August 1998, the Fightinghawk would continue the long Argentine A-4 story.
As a side note, not long after the Fightinghawks entered service, Argentina managed to find more funds and did the obvious, buying a squadron of F-CK-1s to serve alongside the A-4AR with the Fuerza Aérea Argentina.