I have tried to figure out which major scale related arpeggios are best when playing leads like how Django himself did, with two fingers. I have come up with two basic arpeggios that you need to know to develop this technique. The first one is a Cmaj7 arpeggio that is very two finger friendly when played in the two finger decending pattern that I show. The second is a C6 pattern that is also perfect when played with just the first two fingers on the left hand.

—————–7-8–|-7-8—————————
—————8——|—–7-8———————–
————-9——–|———8-9——————-
——–9-10———-|————-9-10————–
—7-10—————|——————9-10-6-7—–
-8——————–|—————————7-8-
———————4-5-
—————–4-5—–
————-4-5———
———4-5————-
—–6-7—————–
-7-8———————

Even though these are both major patterns, they should not necessarily be used over the same major chords. Make sure that you are aware of using the 6th and the 7th and its effect over the top of the chords that the rhythm player is using, which are quite often EITHER maj7 chords or maj6/9 chords.