The problem was: OSX i386 builds used --dist-target=leopard
which forced the deprecated 10.5 CoreAudio API.
(build-host config has been changed now).
There is currently insufficient support in ardour’s
CoreAudioPCM::create_aggregate_device() for the old API,
COREAUDIO_108 needs to be defined or aggregate devices stop
working on OSX 10.8 or later.