Assuming that your charging cct also runs through your switch.. and that your switch is definitely "make before break".. then I would use the engine battery for the blower and to start the engine, leave it switched in for a specified charging time, and then switch to charge your house bank independently when you have the opportunity. I wouldn't charge to "both". Charging seperately gives you more control over where the energy goes.
Under normal conditions, starting the engine takes very little energy and does not need a long time to recharge.
House loads can certainly drag the battery down and so the house battery will require more charging time to replenish it. Use the house battery for all loads incl. instruments any time the engine is not running. Protect the starting battery from being run down at all costs. If you can start the engine, you can recharge.