Sand isn't a very good substrate for ball pythons, and it will absorb a considerable amount of heat due to it's density. You could try switching to newspaper(it's cheap or free) and see if your temps level out better. If the temps still aren't where you want them it would be best to start with one heat lamp and see if that is sufficient. I like to use a 75w or 100w red bulb with a light dimmer, generally I have the light at about 50-60% power, but the dimmer allows me to further customize the bulb to suit my needs.