How are coupled runners handled in exchange wagering?

The short answer is: they're not.  Betfair does not offer exchange wagering on races that include coupled runners.  The problem is that liquidity is pooled between the U.S. and the U.K. and the two countries have differences in regulations regarding handling of coupled entries.  So there's apparently no way for the exchange to operate in a way that satisfies the regulations of both countries with regard to coupled entries.