I reckon there's pluses and minuses with each way of doing it. I tried four different ways and will stick with alternating the ring finger or wrapping my thumb (and I have really small fingers so it can be done)

1 Fretting the two simultaneously is ok but I'm not sure I could change to it at speed and be sure of doing it consistently and without loads of buzzes.
2 Alternating seems the easiest - especially if you are doing bass runs too.
3 Fretting the 6th string with the ring finger and then flattening the joint across the 5th too is possible but tends to mute the 4th when I do it
4 Thumb is ok but does tend to contort my hand so I catch the 1st string first fret with my first finger as my hand swivels.

If that's not clear there's a short video here of my quick attempt. It's about 1.35mb but shows each example above and the problems I encountered.

If you can't play .flv files (work with realplayer) download VLC media player http://www.videolan.org/ which plays most things and is open source and little.