If it's a small tube, you can buy or make a square block collet to hold
it. Line up the existing hole to center, tighten the collet on the tube,
then flip it over and you will be 180 deg opposite for the new hole.

If you plan to do a lot of this, consider an indexer with a collet or
3-jaw chuck.

Miki wrote:

I tried searching, but couldn't seem to come up with the correct search
term. I'm quite sure this has been discussed though.

If you have a tube with a hole drilled in it radially, and want to
drill more holes exactly in-line with the existing hole, how would you
do this on a drill press? Finding the center of the tube is the easy
part, but how do you know you have the tube "clocked" correctly on the
V-block?