When two lectors are assigned the Sunday readings and one of them doesn't show, is it all that catastrophic?
In some parishes it may not be a big deal to simply grab any lector in the pew 10 minutes before Mass; but in others with higher standards, it's a practice to avoid.
When a lector doesn't show up or get a sub, it can put a huge strain on the sacristan to find a last-minute replacement. And when one can't be found, it' can be an added strain on the remaining lector to do both readings plus the intercessary prayers when required.
In addition, when a lector has poured his or her heart into preparing their one assigned reading, to take on additional readings can dilute their focus and knock the wind out of their sails.
Scrambling around to find last-minute lectors is also a disservice to the parishioners, especially to those who come expecting to hear the word of God proclaimed well. When a replacement can't be found, one lector doing all the readings can also appear odd to the parishioners when they're used to having God's word read to them by two different and well-prepared lectors.
Of the many parish websites I've scoured, the most that's said about lector no-shows is what to do about it after it happens, but little about the importance of avoiding these situations before thay ever come up.
In one of my past churches, our pastor would fire any lector or Eucharistic minister if they didn't show up or get a sub. A little extreme, perhaps; but a good policy in any church might be to give one or two (at most) warnings to repeated offenders before removing them from the schedule.
But how does a lector coordinator know who shows up and who doesn't when they can't be at every Mass? They make friends: lots of friends with sacristans, ushers, greeters and other lectors who give feedback on no-show incidents. It works well for me and will for others too.