It depends how you go about the mixing. If you have a mysqli resource and try PDO methods or vice versus, it will error. If you use both but keep the resources and their associated methods in order, you may not get errors, but the code will be prone to confusion that might result in errors.
For example, it would be possible to use mysqli for a SELECT and PDO for an INSERT, but why? Two resource connections might not be disastrous but seems totally pointless to do to me.
The only exception where I'd want both is if I were refactoring code a bit at a time as I had the time to modify code I didn't have the time to change all at once.