These two notes, the third and seventh, are really "intervals" since a chord requires three tones minimum. Since the bassist is responsible for voicing the root, the the rhythm section will fully outline the harmony for the listener.
This chart has a range of these "shell voicings" to play. If you have to comp for a bar of A-7, you just find an A, which is represented as the white dot. You then find the nearest flat 3rd and flat 7th and play only those notes to comp for that bar.
Going from a 2 to a 5 is easy with these voicings. When you move a minor seventh down a semi-tone, you will be on the third of the progression's V7 chord. The minor third of your two voicing will turn into the minor seventh of the V7. Just by moving one finger, you can change from a ii- to a V7.
Here is a link to some flashcards to quiz you on the 3rd and 7th of maj7, mi7 and dom7 chords.