If I write an unsuccessful grant proposal with one PI, can I reuse those ideas on a different proposal with a different PI?
Generally speaking, it is fine to seek multiple sources of funding for the same project.  However, you cannot accept multiple sources of funding to do the exact same thing without permission.
You should check with the program managers of the funding agencies.   Program
managers say they spend much of their time looking for duplicate submissions and rejecting them.  However, if one application is to a government agency and another application is to a private foundation, that might be fine.  Or if the applications are to governments of different countries, that might also be fine.
The PIs should understand that it is normal to seek multiple sources of funding.  But tell them what you are doing and ask them if they are okay with it.