I'm not really clear from your post if you pressed the whole half gallon or not. I made all my tinctures in half gallon jars too. I only press the amount I think I'll need and leave the rest tincturing until the next full moon. I never at that point add more herbs or alcohol to the tincture.
There are double tinctures but I've never done one.
From the SYL manual:
Some of Richard's students make a double-potency tincture. They first make a regular tincture. Then, before using it, they add fresh herbs into the tincture as if it were clear alcohol, and then they let that continue to soak. When that is pressed, you have gone through two soakings with two different batches of fresh herbs. What a brew!
Once you've pressed all the herbs you'll want to throw them away because all the goodness is in the alcohol. I believe what I would do is just start fresh on the 'New Moon' with new herbs and new alcohol.