That's a good question, and the answer depends on your objectives. For maximum warming comfort, you may want to put the heaters in the two rooms where you spend the most time, excluding the kitchen.
For a little extra warmth in the kitchen when you're cooking, you can simply turn on an unused burner on the stove. But be sure to turn it off when you're finished!
If your objectives include mold prevention and energy conservation, you may want to put both heaters downstairs. Because hot air rises, the downstairs heat sources will have a small warming effect upstairs too. And the average temperature of your house will be a bit higher.