I have also logged time in single crew aircraft flown as multi crew. LASORS section regarding issue of the ATPL covers this issue. To count the hours in the logbook the CAA will require the operator to have a valid multi crew AOC. This requires:-
-A valid reason for operating with more than one crew i.e high workload due to the nature of the operation such as ambulance work, calibration or other special missions
-Multi-crew procedures in the ops manual
-The aircraft must have similar instrumentation both sides of the cockpit. Don't know about the Caravan but this would rule a Seneca or Islander out
-Pilots to have a valid LPC/OPC and meet all other experience and recency requirements
Now, as the Caravan is VFR only I don't think anyone is going to be able to justify operating it multi crew, in the UK at least; meaning unloggable hours. If the hours were logged abroad, expect the CAA to ask for evidence of a proper multi crew operation, probably the AOC. Hope this helps!