The jiucier one was probably the point, which has a higher fat content.
When I cook there are 2 things I go by to decide if something is done - one is the internal temp. I shoot for around 200*. The second, and probably more important (for me) is how the meat feels when I slide a probe into it. If there is resistance I don't care what the temp is - it's staying on the pit. Should feel like a hot knife through butter - nice and easy. Having said that - each piece of meat is different - and you will get a tough one every once in a while.
I am glad to see you will be doing another brisket - have to say they are one of my favorites. For the half you froze - since it wasn't as juicy as you would have liked you may want to use that for chili - nothing like a nice smokey brisket to take your chili to the next level.
One question though - why did you cut it in half?