I think you CAN do a better job cleaning/lubing a chain by taking it off the bike compared to doing the maintenance insitu, but for me, the hassles of breaking a link each time and re-threading the chain is more trouble than it is worth. Having said this, I do occasionally take my drivetrain apart (every year or so) for a thorough cleaning.
As to having a 2nd chain so one can be on soak while the other is in use, I doubt a chain that soaks in solvent/lubricant overnight does that much better than one soaked for a few minutes while you're doing other bicycle maintenance. Seems simpler just to remove it, clean it, lube it, and return it to service and not worry about a 2nd chain.
- Mark