Does a plaintiff attorney have to tell a defense Attorney if he is using an ex employee of a company he is suing.

Under both the Nevada Rules of Civil Procedure and Federal Rules of Civil Procedure, each side in litigation is required to disclose the identity of any and all witnesses, as well as persons with knowledge relevant to the action, and the information which that individual is expected to hold.  Under N.R.C.P. 16.1(a)(1), for cases in Nevada state district courts, "a party must, without awaiting a discovery request, provide to other parties: (A) The name and, if known, the address and telephone number of each individual likely to have information discoverable under Rule 26(b), including for impeachment or rebuttal, identifying the subjects of the information."  Therefore if you were hoping to secretly collaborate against your former employer without them knowing, you should be prepared for your information to be disclosed.