The Australian and New Zealand Governments are to hold their first-ever joint Cabinet meeting, in Sydney on this Friday 24 August.  The political parties in charge of the two countries are currently of opposite hue:  Labor in Australia, and National in NZ. 
In some respects, the only surprise here is why this took so long.  For a period  before it was self-governing, New Zealand was a dependency of the British colony of New South Wales, and indeed NZ achieved self-government four years before NSW did (1852 vs. 1856).  The preamble to Australia’s Federal constitution mentions NZ as one of the founding states, which would still provide NZ fast-track entry to the Federation should it ever wish.  Immediately following Federation in 1901, both countries had cabinet ministers born in the other country, and New Zealand cabinet ministers (along with those from Papua New Guinea and from Norfolk Island) are now regular participants in the various Ministerial Council meetings of COAG, the Council of Australian Governments, the Australian Federal-State body tasked with co-ordinating policy.   (As a consequence, COAG meetings, which rotate locations, sometimes take place in NZ or PNG.)  The two countries have agreed freedom of trade in almost all products and services and freedom of movement (at least for each others’ citizens), and have even talked about a common currency.  They have shared defence activities since at least the joint ANZAC force landed at Gallipoli, Turkey, in 1915.
Apart from actual political unions, such as the USA and the EU, I wonder what other two political entities have this degree of co-ordination.  Even the British-Irish Council of the Isles, which links the various national assemblies of Eire, Great Britain, Guernsey, the Isle of Man, Jersey, Northen Ireland, Scotland and Wales, does not involve much substantive collaboration.  No doubt different languages make joint cabinet meetings difficult across many borders:  The only example I can recall in recent years were the joint Franco-German cabinet meetings held under Francois Mitterand and Helmut Kohl.