How to finance when old car is still available?

Hello, I have done a little bit hard with the title award. I hope, because you can still see what I mean at first glance. The following: I would like to buy a new used car. This should be financed. Now I have found a great offer from private, where I would really like to hit. In order for the monthly rates for the new car to fit into the frame, actually the old car would have to go away first. However, the potential new vehicle can be sold any time. We can spend 230,-€ per month on the instalments. The average amount of credit is about 17000,-€. Now I have already calculated various car loans, but always without a final rate. The new vehicle would cost about 21500,-€. That is, without selling the old car I have a difference of about 4500,-€. How do you solve your problem? such a dilemma? Could your banks/credit institutions recommend working with a closing rate?