Cool Ventilated and heated seats, can both options be used at once?

The simple answer is absolutely yes, (and there are various reasons why you might want to in certain conditions. If the air is warm but your seats are still cold, you may want to pop the seat heaters on for a short while to heat up. Or for people who have circulatory issues, a seat heater may be essential even though you want to keep cool air.)

All turning on options does is add some load to the vehicle, and all vehicles these days should be specced to cope with everything being turned on (or in some rare cases will inform you if the load is too high)

Of course, if you have a failing alternator or battery you may encounter issues, but those are the fault of the failing component. Extra load just exacerbates them.