1/2" and 3/8" is that much difference. Even if you change them both to 1/2" you will not likely get the same amount of flow out of both lines. There are other factors to consider such as length, bends, turns, or elbows, and even height. The only way to adjust or control the flow is by adding some type of valve on each line.

I'm guessing when you say 3/8" line you used some sort of plastic tubing. Which if you did, you could add something like a clamp to partially restrict the flow instead of cutting and adding ball valves. Not the best way but......