If you want to save yourself a lot of grief and frustration invest in a hydrometer. About 20 bucks. A good thermometer will get you close but to be positive you are making syrup you need a hydrometer. And you don't need two thermometers. I hope you are using a digital one, all other styles of thermometers are too hard to read accurately.

Check the boiling point of water by throwing some water in a pan on the stove. Check the temp. Write it down. Add 7 degrees and this will get you close to finished syrup. So, if you have a boiling point of 212 for water (about average but remember it will vary even throughout the day so dont use a temp taken at 7AM for finishing at 9 PM at night) add 7 for a temp of 219. Why 7? The boiling point of syrup with a standard density is figured by adding 7.1 degrees to the temp of boiling sap. Also invest in the North American Maple Syrup Producers Manual if you haven't already. Its a wealth of information. Best of luck!

Good luck!