Both applications are capable of syncing to jack. Set both to do that, and
they will sync together.
In ardour: windows->options->sync->Positional sync = Sync with JACK
In hydrogen: in the main window, there is a "jack transport" setting which
should have a green "light" in it.
Sampo
Quoting ish(a)sarai.net:
Can one sync Hydrogen with Ardour so they can play
together( ie Play and
stop together) i.e. we use the mixer of Ardour(with one track of
Hydrogen)
for the final mixdown with the Ardour Mixer.
<>ISh