I would be surprised if you could get a loan on a new house, even with co-signature of your spouse’s parents, while still holding the paper on your current home. Some loans don’t want you renting out the property backing the mortgage, so it gets tricky when you admit that up front as you are looking for yet another (primary) mortgage.
If your inlaws have the means to co-sign, one option to consider is to have them buy the new property outright. Rent the new property from them and rent your current property out until you sell it. At that point you can buy the new property from your in-laws. If they are not in a position to do that comfortably, they should not be co-signing the loan.
A co-signature, for the most part, is total responsibility for the loan should you default, and it maintains that responsibility until the loan is satisfied. For a 15-year mortgage, this is 15 years or until re-finance. It’s way more responsibility than “just for the name.” It’s responsibility for “the loan.”
A credit rating only means you have a good credit history. It doesn’t mean the market is going to stay strong. There are still a lot of voices out there predicting another 10-20% drop in home prices over the next two years or so, and if I were co-signing for that loan I’d make the assumption I just bought the house personally. Therefore (and especially if it’s a great investment), if I’m your in-law I’d just buy the sucker and let you rent it from me indefinitely until you got your current home sold.
It’s my personal financial philosophy never to buy a second home contingent upon an expectation I’ll sell a current one.