I own a duplex, live in one unit.It has two separate street addresses, but the mortgage interest, etc. is apportioned in a lump sum to the whole building.When it comes to apportioning expenses such as mortgage interest, should I split it 50:50, or, for example, proportionally based on the number of bedrooms in each unit (one unit has 2 bedrooms, one has 4, so it would be a 33:66 split)

If not paying rent I would document damages, repairs, or any other expenses such as mortgage you must pay on their behalf as a proportion to use.

If both heaters are on "low", you'll probably get away with it... as long as there isn't too much other stuff on that circuit.As a comparison, one space heater on "low" is equal to about 53 compact fluorescent "60 watt equivalent" lamps, or 77 LED "60 watt equivalent" lamps.

I guess in a sense you’re still following your valuation tools, just offering much lower consideration in proportion to the market value of the underlying asset.

Snow shoveling responsibilities cannot fall to our aging members - not that we get snow in Brockport proportions!

Unless they increase the appraised value proportionally to the percent increase in NOI?

Then ask, are there any sold quadplexes of similar proportions?

Typically returns are proportional to risk.  

Hopefully that leaves you able to save a large proportion of what other income you may have  (if you have one 4-plex assuming you have a job or some other income as well)    That can build up pretty fast when you are able to save a large proportion of it.  

The remainder (assuming it was spent on the new purchase) is attached to the new property.The loan costs are split in the same proportion, and each part is amortized under the respective property.If the old property had remaining unamortized costs from the old loan - those get deducted in full.I know this sounds like too much hassle, but it is the correct treatment.