Kind of on Topic

[Rainbow Six]

The same numbering system is also in operation in the United Kingdom, although of the top of my head I can't think of a current example since the latest round of amalgamations. I presume the Australians adopted it from us.

There were a lot of examples during the Cold War, particularly in the Royal Armoured Corps, e.g. 9th/12th Royal Lancers , 14th/20th King's Hussars, etc, etc. Most of the Infantry ones were in the Territorial Army, e.g. 5th / 8th Battalion, King's Regiment.

In the UK it's largely a mechanism to keep Regimental / Battalion name and traditions alive by having both formations remain on the order of battle and thus technically active. On rare occasions the unit can "de-amalgamate" although I can't think of an example of that of the top of my head - you'd probably have to go back to World War 2.