The thing is also it may just be easier to find an ATP rated pilot or a multi flight instructor that is willing to fly your plane. Then you are just paying them to solo you so you can practice. I believe there are provisions that the instructor doesn't have to be in the plane for certain circumstances they just observe. Getting a multi engine endorsement? Basically. To your private pilot certificate. The FAA may be able to sign you off to fly "YOUR multi-engine plane only" some how also. Some letter of authorization or waiver. Just spitballing but I bet there is a solution to this that is acceptable to all party's.