Is the total damage dealt from a source effectively parceled out across any number of assets?
This is detailed in the Rules Reference Guide (relevant section below, from page 7 under &quot;Dealing Damage/Horror&quot;), but the short answer is that damage is allocated in two steps - first it is assigned and then it is applied. The only restriction for assigning damage is that you cannot overload it (so if you have an ally who can take 1 sanity and 1 damage, you cannot put a 3 sanity tokens on it).
But yes, otherwise, damage or horror can be split among your investigator and any assets as you deem fit.

When an investigator or enemy is dealt damage and/or horror,
follow these steps, in order:

Assign Damage/Horror: Determine the amount of damage and/or horror being dealt. Place damage and/or horror tokens equal to the amount of
damage and horror being dealt next to the cards that will be taking
the damage/horror.
Apply Damage/Horror: Any assigned damage/ horror that has not been prevented is now placed on each card to which it has been assigned,
simultaneously. If no damage/horror is applied in this step, no
damage/horror has been successfully dealt.