If your duplex sits on one lot, then it's considered one property. Therefore, you own half of the lot. The only way you can get equity out is to have the other parties agree to quit claim the title to you, you refinance it, take equity out of it and then stick them back on (if you have to) or buy them out of their equity.
Financing is not done on the structure itself (unless it's a condo), it's done on the property that it sits on. It's like a house sitting on a lot with an attached garage. You can't sell the garage without the house. It's all or nothing.