Automatic transfer switch VS Using a Reversing Contactor with Circuit Protection
If you are building this for sale probably needs to have the label of an nationally recognized independent testing laboratory such as UL. If you don't have a relationship with such an organization that allows you to label control units that you build, you can't meet that requirement.

If you can get it approved, you still have the question as to whether or not it will work. Directly connected motors will have an unacceptable level of stress from an unsynchronized transfer. A VFD should be able to withstand the transfer, but will likely shut down. Many VFDs have features that may be able to prevent a shutdown, have only a speed variation, or shut down and immediately restart automatically.