trade in 2 cars for 1

You will get a much better price if you sell your cars yourself.

I would first try to sell the cars on CarGurus and Craigslist - both are free services and get a lot of traffic. However, if you really want to trade both in, but are unsure of what a dealer will give you, take them to CarMax and get a trade in price. CarMax will take the car even if you don't buy from them. They would likely give the best price for a newer car ready to sell, like your 2015 Prius.

Thanks. I will try to sell them both on my own