Playing notes in the same fret but on different strings can pose a problem for many people. There are a few approaches to solving this problem. Some people say that you should always use a different finger for each note, but I have found that to be quite hard and often impractical. The rolling technique has some flaws, but in my opinion is the best method, and is used by many people.
I would recommend watching the video before reading this lesson text, it will make a lot more sense!
The most important part of this exercise is to make sure that you mute the string that you are leaving. The example that I show in the video uses the notes G and C, found on the 4th and 3rd strings, both at the 5th fret.
To get the basic idea you should position the tip of your finger (I call this the point) on the note G (4th string, 5th fret) and play the note. Then roll onto the flat of your finger to play the note C (3rd string, 5th fret) making sure that the tip stays in contact with the 4th string to mute it, remembering that you only have to touch the string very lightly. The mute is VERY important.
Then roll your finger back to the note G, making sure that the underside of your first finger stays in contact with the 3rd string to mute it.
You should practice playing these two notes with your first finger and working on the rolling action and making sure that you are muting the string that you are not playing! This will probably take you a while to get right, just be patient and keep working at it - it is worth it! Once you have that sorted out, then try doing the same thing with your third finger at the 7th fret. Remembering to keep muting the string as you leave it.
Using the Minor Pentatonic to practice the technique
A good way of putting this into practice is to apply it to the A minor pentatonic scale. Some of the notes do not need the roll but it is good to get it into a "real" situation in a way that you might actually use it. Use the standard fingering for position 1 of the A Minor Pentatonic. Here is the TAB:
Once you get that down you might like to try this little trick that I think sounds pretty funky, where you reverse every second pair of notes. This makes for some cool licks too and is quite a challenge :)