Originally Posted by Doppleganger

They didn't. And moreover, really had no way to co-operate in a practical sense. In fact, I'm not even sure why they were 'allies'.

They did. Up until the beginning of 1944 there were dwindling numbers of voyages by surface blockade runners and commencing 1943 voyages by U-boats and even before Italy withdrew from the war voyages by Italian submarines to Penang. After Penang became too dangerous voyages set out for Djakarta/Batavia. As late as November 1944 U-219 and U-195 arrived with radars, jet and rocket engines, blueprints etc. Among their cargo were twelve broken down V-2 (A-4) rockets.

In a 1982 TV interview former general Kawashima indicated that 2000 kilograms of Uranium oxide were successfully shipped to Japan.

In his memoirs of armaments Mnister Albert Speer a flight to Tokyo via the polar route with a Junkers Ju-390 was described.

Originally Posted by 84RFK

I beg to differ...

There was more contact than we are aware of, as both merchant raiders, their support vessels, and cargo-subs reached port in Japan throughout the war.
Blueprints, planes, engines, and other technology reached Japan in a more or less dismanteled state, and while it did contribute to the Japanese war-effort they had little to no impact on the warfare itself.

Among the U-boats known to have reached port in Japanese controlled waters are:
U-181, U-195, U-219, and U-511.
At least 3 of theses were loaded with blueprints and technological hardware, two of the vessels even carried dismanteled V-2 rockets.

Several other U-boats was sunk before they made it to Japan, and about 11 boats made it back from Japan, indicating that even more U-boats had initially made the trip from Europe to Japan.

Among the U-boats that didn't make it was U-864 wich was torpedoed by a British submarine in the waters outside Bergen in Norway.
This was the first time in history that a submerged submarine torpedoed another submerged submarine.

You could add several more U-boats. Italian U-boats taken over by german crews too such as UIT-22, UIT-24, UIT-25, U-862, U-861, U-196, U-183 and host of others that i cant recall at present. 47 attempted the voyage eastbound. Many failed but many made it and a few even returned to Europe.

There were also the less successful Yanagi voyages. I-30, I-52 etc

The most interesting issue is that of communication flights...

The Messerschmitt Me261 looked like a much enlarged Bf-110 twin engined fighter. In detail however it had an entirely novel airframe able to accomodate it's crew and between 7-8 passengers. It's role was as a long range courier aircraft for important passengers and documents. The Me-261's first flight was 23 December 1940.

Following Germany's invasion of Russia, Japan which maintained diplomatic relations with Moscow throughout the war objected to Luftwaffe overflights of the Soviet Union to maintain contact with Japan, so subsequent connecting flights were made by civil registered aircraft in Deutsche Luft Hansa DLH registrations.

On 30 June 1942, a successful flight from Saporoshje (near Rostov) to Ninghsia (modern Ningxia) proved a link could be viable. The flight used a Savoia Marchetti S.75 RT (RT = Rome -Tokyo) trimotor aircraft. It's route was north of the Caspian and Aral seas then through the Altai Range past Lake Balkhash to Ningxia.

The aircraft's navigator Dr Publio Magini was of the opinion that had they not been required to land at Ninxia, with the fuel aboard they could have continued on to reach Tokyo. A second Italian flight used a secret airfield near Kabul developed by Brandenbergers as a refueling stop. Probably this airfield was used subsequently for German long range flights too.

A Tachikawa Ki-77 transport attempted a flight to the Crimea from Singapore but the British were tipped off by ULTRA and shot it down over the Indian Ocean.

Subsequently in early 1944 Kommando Nebel was formed using civilian test pilots and three Ju-290 aircraft converted to civilian airliners with extra fuel tanks. Flights are suggested to have departed Bulgaria via Kabul to Ninghsia.

At least one polar flight to Sakhalin was flown by a Bv222 Wiking flying boat.

The last such flight is mentioned by Armaments minister Albert Speer in his autobiography flown in a Ju-390 with test pilots flying via the "Polar Route"