Do today's cell phones have any option to do that?
None that I've seen. Typically the A carrier is TDMA or GSM, and the B carrier is CDMA, so if you forced the phone to the other one it'd have to fall back to analog and cellco's don't like that.
Ten years ago when I had an analog car phone I would force it to switch all the time when I was on trips because I knew that the non-default carrier had a better roaming deal.
R's,
John