It is very difficult, if not impossible to guess what your surgeon may have been thinking or was concerned about when he/she advised against combining the procedures. It may be that you have a medical condition that would increase your risks with longer procedures, or that he/she thought the technique planned for one procedure may compromise his/her ability to successfully achieve your goals for the second area of concern.
We all have our individual levels of comfort with what we can accomplish safely for any given patient within a given period of time... some surgeons take longer than others to do the same job. That's not a judgement of quality, just a fact.
If having the procedures together is important to you, I do think this may be possible and safe, but to be certain you should visit a few more surgeons- make sure they're Board Certified by the American Board of Plastic Surgery and that the facility where the operation will occur is certified.
After gathering recommendations from a few reputable surgeons, you'll know what is in your best interests.