At the time you could have pressed Shift-Record which would have picked up the recording right where it left off. That's Append Record.
What you can do now is select the Time Shift Tool (two sideways black arrows) and push the second track later (to the right) until its beginning matches the first track's end. Audacity will smash them both together when you export.
I think there's a way to automate that, but I gotta look that one up.