RF after Ultherapy. Would it enhance results or have any negative effects?

Without more information it is impossible to determine whether you are likely to experience clinically significant improvement from RF treatment.  While this may enhance your ultherqpy outcome (and certainly won't hurt your Ultherapy results), you should ask your physician what you may or may not experience after RF treatment.

I have not seen, nor heard, of any problems treating the face and neck with Ultherapy and then waiting six months to treat the same area with external radiofrequency (ThermiSmooth or Pelleve).  The six months allows for the skin tightening to occur before performing a second treatment on the same area. Increased collagen production has been proven to occur for at least six months after the treatment. An exception would be to use the ThermiSmooth on the same area if the 1.5 mm transducer is not used in Ultherapy. If the 3mm. and 4.5 mm. transducers are used, their energy would be deeper than the heat delivered by the ThermiSmooth, so ThermiSmooth could be used to treat the crepy texture of the skin the same day as Ultherapy treats the deeper tissue planes for skin tightening.                                                                            The information provided in Dr. Shelton's
answer is for educational purposes only and is not intended
to constitute medical advice. The information provided should
not be relied upon as a substitute for consultations with a qualified
health professional who may be familiar with your individual medical needs.

Adding RF to ultherapy can help increase collagen in your skin and is a good idea.There is no downside to doing both.We often combine treatment modalities to achieve the best results.