The examiner or investigator should look at the furnace or water heater and make sure it is in proper working order or ‘modified’. CO gas could kill both and in their sleep that is why they are still in bed. It could still be a possibility.
Or the care taker done them both in somehow
Who knows until an investigation and autopsy is complete, if one could be.
Without blood or soft tissue, a lot of things will be undetectable.
Depending on the age of the furnace a number of things could be done temporarily that there would be no way to discover later. (close a flu damper for instance).
But on the subject of CO and the time of year. Everyone that heats with gas or wood should have a CO detector in the house. Mine saved me and my family.
My step daughters friends nearly died because they didn’t have one.
Fingernails, teeth and hair can be tested, even on a decomposed or mummified corpse.