Knowing when to sell and when to hold?

[Vincent Plant]

I have both sides of a twin that I have owned for about 3 years now. They are rough to say the least. In a rough area, in rough condition, low income tenants that don't take good care of the property. 

All in all it's been a pretty good run with them. They cash flow nicely (when the tenants pay on time). I got a great deal on them. My mortgage is low, the maintenance has been much more manageable than I expected, and in my 3 years of ownership they have appreciated considerably even with all the negative factors. 

I had the boro go through them recently and they are in really bad shape. I know I am going to be facing a lot of large expenses. I know the way the market sits I can unload them as-is and make a nice profit on them. But do I want to sell off 2 homes? I have always heard the saying never sell an asset when building wealth. And what am I to do with the proceeds? I could never buy homes now for the price I paid for these. 

Should I grind through and try to maintain/fix these places up? Or sell off my class D assets to try and upgrade to better properties?