No such thing as a free lunch.
I have such "a" property. Two detached SFR on one lot, one behind the other, with separate adresses. The only thing they share is a water bill. When I look at the tax info, it's as if the back house doesn't even exist, except that it's zoned for two families.
Built between 1890-1900, it is considered an approved zoning variance, meaning it doesn't comply with today's zoning, but the city can't do anything about it. There may, in some areas, be limitations placed on what you can do with such a deviant property. I have been told by local inspectors that the city would prefer the back house be torn down, and that I would be unlikely to get any building permits approved (in essense, I would be prolonging the life of a building they would like to see go away).
It is unlikely that you would be able to split the property, unless it satisfies current street frontage, setbacks and minimum lot size requirements, and asking for an exception would be a very long shot.
You can't look at this property's ARV as if it were two sparate properties. It is, essentially, a duplex, and the zoning should reflect that, and when looking at comps, should be compared to other duplex properties. That they are detached should be considered, but I have found it's not material.
I'd check with local zoning and building inspectors to get their take, and check for any existing code violations and any potential obstacles in the "R" part of your ARV. The best way to determine what you can expect is to go to the source. Worst case, if you still want to move forward, is you'll have ammunition to argue a lower price if you do find limits on what you can do with the property.
In the FWIW category, the second house, since it is behind and away from the street, is an really easy rent. Mine has a driveway leading to parking between the two and the back house is quieter and secluded. The downside is trying to sell. The unusual and "grandfathered" nature will raise questions and doubts, just like you're doing now, and there will be those investors who will just not consider such a property in this market for those reasons.
Of course, you experience may be different. Just don't scrimp on the DD, and if the local authorities do frown on the property and will make life difficult, it's a good argument for a significant discount.