You are absolutely right! you can combine both for improved results.
Basically the two treatments target different layers of the skin. Dermal fillers are at at deeper layer, and serve to volumise your skin and reduce the appearance of "sagging" while Rejuran is given intradermally to improve conditions such as pigmentation, general skin quality.
It would be best though to consult your doctor and see what else he would recommend based on your skin needs. Skin boosters may be used on a separate occasional for improved hydration status too.
Dr Winston Lee