Ip mobility is for when clients roam between controllers.
Frankly, having two ISPs for a single deployment is a poor design from a management, troubleshooting and performance perspective. Clients that roam to the second ISP would have to reestablish all of their connections. When users complain and have issues, you would have almost no way to determine if the problem is the client device, the wifi or the ISP, complicating your management of such a possible solution.
They should have all clients on the same ISP. The ISP should be sized correctly and be done with it. - That is my advice.