Point your antennas, 1 at a time, for optimum reception, prior to combing their signals.
Connect equal length cables from your antennas to the antenna inputs of the JOINtenna.
If you are going to use a preamp, use a short coax cable from the JOINtenna output, to the preamp input. Note; Do not amplify the signal between the antenna and JOINtenna. In most every case, it would be problematic. Only combine antenna level signals.
Please note, when combining antennas, there is no guarantee that it will perform as well as you'd hoped. e.g. Both antennas can receive the same signals. Antenna 1 is receiving a good quality signal on CH15. Antenna 2 receives the same CN15 signal but at a fraction of a moment, 'out of phase' with the first signal. This can cause multi-pathing, which ruins the Signal Quality, making that signal unusable. If this does occur, you may have to run another cable to one of the antennas to get all of the signals into your structure. You can connect the two cables to a coax switch behind your TV.