Hi Matt,
Thanks for visiting the Disney Parks Moms Panel with your question. What a great vacation you are planning!
Disney's Old Key West Resort is one of my favorites. The theme of the Florida Keys is so relaxing. I also think that you can never go wrong with a spa treatment while on vacation, and
Disney's Saratoga Springs Resort & Spa is a wonderful place to have one.
There is not transportation directly from Disney's Old Key West Resort to Disney's Saratoga Springs Resort & Spa, so your wife will need to transfer at some point. One relaxing way to do this would be to take a boat. Both of these resorts are connected by boat to Disney Springs, so your wife could take the boat from Disney's Old Key West Resort to Disney Springs, and then catch a boat from Disney Springs to Disney's Saratoga Springs Resort & Spa. While this will be a relaxing trip, it might take a while as she will potentially need to wait for boats twice.
If speed is the key, I suggest taking a taxi. While this will cost a bit of money out of pocket, it will be the quickest way to get from resort to resort. Your wife can head to the Hospitality House area of Disney's Old Key West Resort, and if there is not a taxi waiting, she can ask one of the cast members there to call one for her.
I hope that these thoughts are helpful for you, Matt. Have a great time planning your magical vacation, and please do let us know if you have any further questions.