( Suppose the cash flow is $400, it means it would take me $15000/$400 = approx 3 years to get back my $15k) That would mean hoping the price of the property increases because of the flooring.

Just between the increased Vacancy amount and insurance I believe it will wipe out your Cash Flow projection.You will find it in your best interest to stay conservative with your numbers.  

If we increased rents to match those (after renovation, of course) the cash flow would be $750/month.

My instinct suggests: sell both of them, and put it all towards that forever home that may very well become increasingly out of reach if you don't.

(Rule of thumb suggests that falling below 1% gross return per month makes it increasingly hard to cash flow positively on average, when borrowing to the max).A similar question: Are you getting a bargain* (compared to sold comps)?

The upper unit is fine, and no walls need to be opened.The electrician says it would costs around $8k to  put separate meters , panels and rewire upper and lower unit as needed.By separating the electric, by how much would it really increase the value of the two family?

Personally I would flip houses Each time increasing the investment cash I have until I could buy an apt complex to replace my income

Ideally the property would have one meter that all the units share, and one solar system that feeds that meter.Lastly, leasing is increasingly less attractive to new solar customers. 

If a MFR produces 10k of operating income and is valued at a 10% CAP rate, the value is $100k.Assuming the CAP rate holds relatively constant, the value won't increase until the cash flow increases. 

However, NY state law limits the increase in assessed value is capped at 6% in a year or 20% in five years.