Well, I wouldn't go so far as to say "never" use two fingers to do bends.
I agree that you don't have as much control, especially where vibrato is concerned. There's also the possibility that, as a beginner, if you constantly use two fingers, you could get into the habit of always doing it when you don't need to.
If you find after some time that you just can't do bends with one finger, no matter what you do, then feel free to try using two instead. If you're using a heavy guage string or playing on an acoustic you may have no choice.

One thing to remember about guitar, is never say 'never' to anything.
Just because somebody says it's not a 'proper' technique doesn't mean that it won't work for you. Hell... even if you have to use your nose to play something, if it works for you and sounds good, then that's what's important.

Just as a fer instance...when I first started playing, I was told in no uncertain terms "never wrap your thumb around the neck when you're playing... always keep it anchored at the back". Thankfully I didn't pay any attention cause I use that technique all the time now and find that in some cases, it's the only way to play certain chords.