Legal? Yes. You can not get arrested or be given a ticket for doing it.
Approved by the AMA? No

As the rules are written right now you may only fly a turbine powered model if you have a turbine waiver. Buddy boxes are not specifically dis-allowed or allowed, some have interpreted this to mean that it is OK but several years ago the story I got from the AMA was that it is not allowed unless both pilots have a turbine waiver. You may not use a turbine powered airplane for a turbine waiver check flight right now either.