No you definitely should not do this. I can't think of any specific documentation that points this out, but each instance of SSRS or PBIRS should definitely have it's own set of databases. You can migrate an old SSRS database to PBIRS, but you should not point both instances at the same database.
One reason is that PBIRS gets updated every 4 months and you should always try to keep reasonably upto date as Microsoft only support the current version and the 2 prior versions. These updates often include small changes to the database schema so at somepoint it's highly likely that PBIRS would break SSRS.
Another reason is that the servers are constantly reading and writing to the databases and expect to have exclusive write access. With two instances connected you could cause lock/blocking that could cause strange behaviours on both instances.
