  We present optical, infrared and radio observations of the powerful FRII
radio source PKS0347+05 (z=0.3390), and demonstrate that it is a rare example
of a radio-loud/radio-quiet double AGN system, comprising a weak line radio
galaxy (WLRG) separated by 25 kpc (in projection) from a Seyfert 1 nucleus at
the same redshift. Our deep Gemini optical images show a highly disturbed
morphology, with a warped dust lane crossing through the halo and nuclear
regions of the radio galaxy host, tidal tails, and a bridge connecting the
radio galaxy to the Seyfert 1 nucleus. Spectral synthesis modelling of our
Gemini optical spectrum of the radio galaxy shows evidence for a reddened young
stellar population of age <100 Myr. Further evidence for recent star formation
activity in this source is provided by the detection of strong PAH features in
mid-IR Spitzer/IRS spectra. Together, these observations support a model in
which both AGN have been triggered simultaneously in a major galaxy merger.
However, despite the presence of a powerful FRII radio source, and the
apparently plentiful supply of fuel provided by the merger, the nucleus of the
radio galaxy shows only weak, low ionization emission line activity. We
speculate that the fuel supply to nuclear regions of the radio galaxy has
recently switched off (within the last ~10^6 yr), but the information about the
resulting decrease in nuclear AGN activity has yet to reach the extended lobes
and hotspots of the FRII radio source. Based on this scenario, we estimate that
powerful, intermediate redshift FRII radio sources have lifetimes of t\sim
5x10^6 yr. Overall, our observations emphasise that the fuelling of AGN
activity in major galaxy mergers is likely to be highly intermittent.
