The short answer is…
Yes and No.
Witnesses are not allowed to hear the testimony of earlier witnesses.
If you can get your McKenzie Friend on the stand as the first witness at the beginning of the trial, then after he’s been excused, you can get his help as a McKenzie Friend. If your McKenzie Friend is there while any other witness is being examined, then no he won’t be able to be called to give evidence.
Is this the same in employment tribunal?