My friend Anthony, from another forum, started using the term "roving" gun; very descriptive, as I do not have a consistent place for the secondary weapon, and indeed, might go for it instead of the "primary" weapon, depending upon the clothing, circumstances, and environment. About the only constants are that I tend to always wear a weapon at the 0300 position, and one of my two or several weapons is an SP101 revolver. The SP101 may be the larger, the smaller, or I may carry more than one SP101.
A favored place to tote a secondary weapon is in a cargo pocket of my VertX pants, which I wear on police patrol, and also wear on my own time, sometimes. It is important to note that most cargo pants let heavy objects flop about too much, though VertX pants seem quite good in this regard, but these particular pants rip/tear much too easily, so I do not recommend them. I would love this pants if they didn't tear so readily; I have not found any cargo pants that can so comfortably tote a handgun in the cargo pockets.
With some pants, and some footwear, ankle carry works well for me. This is, for me, a special-circumstances way to wear a secondary weapon.
I have worn a secondary weapon inside or outside the pants waistband, at the "appendix" position, right or left side.
I have worn a second gun directly opposite the primary position, at 0900. This has the advantage of mirroring my primary-side draw-stroke, and takes advantage of the fact that I am actually left-handed, though I have trained to be functionally ambidextrous with handguns. (There are actually some advantages to using one's dominant hand as the support hand.)