Can I mix different cubensis strains in a PF cake?

Yes, you can. But most likely one strain will prevail, so inoculate each jar with its own strain.

Yes, you could.
But there is not really a reason to. Most likely one strain will prevail, if you inoculate the jars each on 1 side you might get 2 strains fruiting from 1 cake.
But in general it's better to inoculate each jar with its own strain.