That's probably fine and if it gets too hot just unplug one. They don't get very hot so I use heat lamps about 12" above the keeper for extra heat.
EDIT; Also you can cover the Vented lid of the keeper with Saran Wrap(Plastic wrap) to help hold heat and humidity in. Cover the lid and cut a hole in the center where the clear access lid is and place any handles through to help hold the plastic down. I usually leave the vents on the ends uncovered and use scotch tape to adhere the Plastic Wrap to the underside of the lid. Works very well.