This question comes up from time to time.
The line is, "of course you can". The reality is "you're damned if you do and damned if you don't."
Across the world food technical people are in short supply and auditors are no different. They need to be trained and observed auditing to ensure they are competent. As they are joining online, perhaps they're only joining for the documentation section? If you're not sure what they will be doing, ask.
The CB will tell you having a trainee auditor is no different. It won't impact your audit and only one person will be auditing. The reality though is you have two pairs of eyes. It is possible that one of two things will happen, one auditor will spot and raise something the other will miss or the experienced auditor will feel the pressure to be absolutely "by the book" rather than advising as they may on their own. Has it had catastrophic outcomes when that has happened for me? No and the best experienced auditors I've known will actually openly say "If I was auditing alone, I would have raised this one additional issue but it's not going on your report as I was not auditing you" for example. So it can work in your favour, but it's rare.
The problem you have is by refusing a trainee auditor, you risk irking the auditor (probably not as much as you might think though but it's possible) but also you're kinda not taking one for the global technical team as pretty much everyone I know is having one of their audits this year with a trainee because we desperately NEED the trainees trained. Trust me an inexperienced auditor on their own is WAY worse.
So I'm tempted to say yes, you can but also suck it up.