It's a little hard to tell exactly how long the tubes are until you take them off. Practicing with a color changing yarn can help if the colors change evenly. I don't think it will matter too much as long as they are close in row count.
To secure the ends together, I wanted a seamless look. After some experimenting I figured out that if you leave the top end open and put markers through each stitch and undo the bottom row to leave open loops, you can match the ends up and weave the tail through to mimic the stockinette stitch and leave no visible seam.
|Line up the pieces so the loops point to a gap in the other piece's loops.|
|Imagine where the missing row belongs by looking at the rows above and below.|
|Mimic the path of the missing row using the other rows as reference.|
One thing I really like about this project is that it helps you understand the construction of your stitches. When I first started knitting, it just looks like a blob of loops and now I can look at my stitches and read them.