Originally Posted by Shawn D.

You don't want the gauge/cluster to supply the opposing force, you want the gauge fitting to supply the opposing force by holding it firmly while you undo/reattach the line's fitting. This is a basic principle for this and other line interfaces like where the flexible and hard brake lines meet. If you do not do this, you can break or weaken the part you are screwing the fitting out of or into.

It's also much easier, especially if you can position the two wrenches like a "V" and squeeze them together -- this works both for loosening and tightening.