Mostly it’s fairly clear which side any given witness’s evidence supports, but once in a while there’s a witness who has relevant things to say, some of which help one side, and some of which help the other. In those cases, both sides may be thinking of calling the same witness.
A witness isn’t ‘owned’ by one side or the other. There’s no rule that once the other side has interviewed a particular witness, you’re not allowed to. In fact there’s nothing to prevent a witness giving a statement to each side.
But there are two important things you mustn’t do in this situation. The first is to ask the witness – or let them give you – information about your employer’s case preparation. That information is privileged, and you are not entitled to it. It’s probably worth pointing this out to them when you speak to them – and make sure they understand that the same holds the other way: they mustn’t leak information about your case preparation back to your employer, either. The second is put pressure on them not to give evidence for your employer.
If your employer finds out that you’ve been doing either, they are liable to make a big fuss about it to the tribunal of the kind that could result in a costs order against you – or even having your claim struck out.