You ask whether it's ethical. Well, the rule in all cases, civil and criminal, is "There is no property in a witness", that is that neither side owns the witness. That's as well, because the witness might have information of value to both sides.And imagine that a witness gives evidence against you. If your side has interviewed the witness and taken a statement, that statement may differ from what the witness now says in evidence. You can then put, in cross-examination, that the witness's evidence differs from the statement and put the statement to them, pointing out the differences. That goes to the credibility of the witness, that they give two different versions whether differing wildly, hence suggesting dishonesty, or merely in detail, suggesting inaccurate memory.
But the witness you see as yours, might refuse to answer (perhaps unlikely) or may say the same as they've said to you.
I can't see any particular reason why the usual rule would not apply in your case. However, enthusiasm for interviewing the witness might attract unfavourable comment along the lines of "Were you frightened what the witnesss would say? Did you think you could influence the witness by reminding them of they're still being an employee? What were you hoping to achieve, given that you knew that the witness was going to testify for the other side?" That line of enquiry could be more damaging to the other side than what they may have found by speaking to the witness is beneficial, if indeed, it is.