We are thinking cost wise it would be easier to have to replace one roof .

Everything has a life-span (from air conditioning systems to refrigerators to roofs).

I dont yet know if any of these properties' major systems (roof, HVAS etc) need immediate repairs and this will factor into my decision.

Also, the both apartments were fully renovated(roof, electrical, plumbing, siding, furnaces, HW Heaters) only 3 years ago.This duplex is also in area where most buildings were built in the early 1900's and have very small bedrooms.

This a 4 unit building constructed in 1916 with roof replaced 3 years back. 2 units has 3 bedroom /1 bath and other 2 unit has 2 bedroom/1 bath + a dinning room.

The good news is that it has a roof, siding and furnace that are in good shape.

The roof of the house looks good but there's a hole in the garage because of a design flaw so we'll have to look at that.

I think you are missing some costs - maintenance new roofs - carpet painting.  

Don't want to sound negative, but I've talked to investors with a large portfolio of houses and they complained to me that profits were eaten up by costs such as replacing roofs, replacing AC's, vandalism or other issues from tenants, evictions, etc.  

I've also done roof, foundation, drywall etc…