Originally Posted by
banerjek
Though there's a good reason as to why you shouldn't.
If you are cruising along in your big/big, downshifting for any reason is awkward. Much better to near the higher end of the small ring since you can at least go both up and down. The only practical use case I can think for riding big/big is because you have good reason to believe that's the lowest gear you're going to need and you expect to upshift soon.
But if you have to do an urgent shift-or-fall-over under load, it's better to shift to smaller gears/cogs, which for a downshift means big-to-small chainring. (I can't think of any reason for an urgent shift-or-fall-over upshift...)
Try shifting to a larger cog under a decent load at really low RPM.