A second uplink is bought for the day the first one fails, and on that day two things decide whether it helps: how long the switch takes, and whether the rules underneath still point where they were meant to. Both are arithmetic, and both are easier to check here than at three in the morning.
Policy routes, in order
The first matching rule wins, exactly as on the console. A rule that covers everything above a narrower one makes the narrower one unreachable.