Your lawyer is not only able to talk to them, he *should* talk to them. Every witness the state expects to call at trial should be interviewed in advance of trial in order to give your attorney the opportunity to know exactly what the witness is going to say. The witness interview should be recorded verbatim by a digital recording device or a court reporter. Having a full transcript of everything the witness has said is a huge advantage that should never be overlooked.
Many lawyers send investigators out to interview witnesses. This is Ok and less expensive than having your lawyer do the interview, but not optimal. Only your lawyer knows all the legal requirements of your case and is in the best position to take a witness interview in a useful direction once the witness starts talking. Even the best investigators will rarely match a good lawyer's skill at witness interviewing.
You should never talk to witnesses without your lawyer's knowledge and prior consent. There is always the danger that anything you say to a witness could be construed as "Witness Tampering", a felony offense. Let your lawyer do the talking to the witnesses.