Hmmm. Seems to me that only 3 relays are required:
1 double pole on the thermostats to potentially close the TT circuits for the boiler and the heat pump when the thermostats call for heat
1 aquastat which opens on low temperature mounted on the wood boiler and in the TT circuit for the boiler. This aquastat prevents the wood boiler circulator from running when the boiler temperature is below the set point.
1 aquastat which opens on high temperature mounted on the wood boiler and in the TT circuit for the heat pump. This aquastat prevents the heat pump from running whent the boiler temperature is above the set point.
Of course the set points for each aquastat should be identical.