this statement is only in part correct. You can do an efficient amplification of both in one reaction but you have to consider some things.
16S is normaly very high abundant
Standard Mastemrixes are not desigend to amplify more than one target especially in a scenario where on template has a significant higher abundancwe
Look for Mastermixes which are dedicated for such a scenario (e.g. UCP Multiplex PCR kit)
I have worked on exactly this situation for more than a year now and therefore know the drawbacks and hickups. In case you need more info plese send me a message