Just to add you want both heaters in the 3W-5W per gal
range. ie: if you have a 55 gal
tank you want both heaters to be 200+W depending on the temperature difference of the room and what you want your tank to be at. Keeping your tank anywhere from 76-82 degrees is ok but make sure the temp stays constant. You do not want temp swing greater then 2 degrees during the day/night cycles. If increasing/decreasing temps do it slowly 1-2 degrees per day.
Getting two lower watt heaters is just going to make one of them work harder since getting them to match up perfectly is nearly impossible. If you work them harder then they are designed to that leads to a possible failure.
As everyone else stated the point of two heaters is for redundancy if one fails, not so they both can heat the tank. You can keep both in the tank, sump, or one in either. Personally I would keep at least one in the tank in case your overflow stops working for whatever reason. (If you have a sump)
I would buy titanium heaters
versus glass ones to avoid breakage issues.
Keep them vertical or horizontal but at least 4ā€¯ below the water level to keep them from being exposed from evaporation. Keep them off the sand. You can place them behind lr
to hide them but I would not let the lr