Contact your airline directly. Most airlines have a "special needs desk" or a "mobility advisor" or something similar. Speak with a human, and ask what the policy is regarding bringing multiple devices.
In the US, domestic airlines must fly your personal medical equipment for free. There should be no fees. You can bring a suitcase full of medical equipment and supplies - just do not include *anything* in that bag that is not clearly a medical device or medical supply - otherwise they can (and will) charge for the bag.
I believe in your scenario, you will be advised to bring both devices to the gate, however it is possible that one of the devices will be checked through with luggage. Regardless of how it happens, both devices should fly free - but do double check that with the airline.
Just a bit of advice: grab some Velcro One-Wrap from
Walmart or
Amazon (I personally like the 1" width) and cut off a strip long enough to help hold each of the devices securely closed while in the cargo hold. Even "gate checked" items are placed in a cargo hold; it's just a hold that is reserved for items that folks brought to the plane door, and luggage that won't fit inside the cabin. Making sure that the devices will stay securely closed in the fold will help lessen the chance of damage. It goes without saying that you should remove any and all items that could get lost or fall off before you arrive at the airport. It can be fun to decorate a mobility device (guilty of it myself) but the baggage handlers are not too concerned with making sure that the battery operated lights are treated gingerly; stuff gets shuffled in there kind of randomly, and so plan accordingly.
Let us know if we can help in any other way - have a great trip!