I am thinking about this because it is an interesting question. The way I would, unconsciously, make the decision as to whether to use a singular or plural verb form would depend on whether I saw the two countries as sharing a single history or having two separate histories. If I thought of British history and American history as being closely interconnected I would probably say 'British and American history is...' If I thought of them as being distinct I might say 'the histories of Britain and America are...'

are

If you say"French and British history IS exciting," then you are saying that the history of (France and Britain) is exciting (ie, the wars they have fought against each other, etc). If you say "are exciting," then you are saying that the history of each country is exciting

Please