Yes you may mix the two together, use a separate bucket pour the contents into the bucket and mix vigorously, this is very important. Depending on the different sheens mixed together will determine what sheen you will end up with, when done mixing put a sample on the wall and let dry to assess the sheen.

Jim
Aardvark Painting Inc.
Chicago Illinois

This is fine. Make sure the paints you plan to mix are the same type. Example- water with water based and oil with oil based only. Never mix the two different types.

Just mixed flat acrylic and oil based gloss, by accident. I followed Jim Andre's instructions, (mixed thoroughly) its fine!!! Dried quick on fresh plaster and drywall, whew!!! Thanks, everyone else here says no way- wrong!!