Sorry, I couldn't think of a more precise title.
In some songs I encounter parts where I play two single notes on the same fret but, for example, 12th fret on the b and e string in fast progression.
This song here is a good example for that:
My question is, how do you put your fingers to play that efficient, fast and clean?
Do you play it with one finger or changing index and middle finger?
What would be the best method to do that?
Why don't you just use one finger for that fret? For instance, in the first line of your tab, use your first finger as a barre on the 12th fret, and play the 15th and 14th frets using your 3rd or 4th finger. That's what I would do.
But using a barre, it would be more difficult to mute one note when you play the next one. If you want to mute it, it would be easier to play it with two fingers (index and middle I guess). By using this way, after you play one note, you can lift your respective finger and the note will no longer sound.
Both of these are solutions. You have to try them out both and see which one you like better. There isn't a definite solution for all these examples. In a song you might see that it is easier to use a barre, and in some other song, it might be easier to use two separate fingers.
this is actually a problem that comes up a TON in sweep picking. players are supposed to play 2 sometimes 3 adjacent notes across different strings in rapid succession while still sounding smooth and clean.
the secret is in the first knuckle of your finger (the one near the tip). let it collapse in the opposite direction it usually bends (be careful if your not used to it) and use it roll across the string the 2nd string to the first. practice getting used to collapsed knuckle first and then see if you can cleanly roll from one string to the next. DO NOT BARRE THE NOTES! only one should ring out at a time with the other being muted by the fat of the finger.
I've personally found this doable with all but the 4th finger with the easiest being the first, then second, hardest third. it takes a fair bit of time to get used to so don't rush this one.
If the 2 strings are next to each other, I use the index and middle finger. If they are not I use the index and the ring finger or pinky. Now for the picking hand, if you are in the second case (strings not next to each other, you will need to practice some string skipping excersices).
Also for clean playing, you will need to mute the other stings with the picking hand (I prefer using the bottom part of my palm to do this).
After some practice you will start to speed up and your sound will become cleaner.
You would barre the fret with one finger. Using two fingers limits your movement to the other positions. You can mute with the picking hand if you are getting unwanted noise.
