Peanut butter is:
comforting, delicious
So having a foot in both jars suggests someone who is:
1. Enjoying two sources of pleasure, affection, or
emotional nourishment.
2. Feeling lucky or spoiled, getting more love,
attention, or joy than they expected.
3. A little greedy in a harmless, endearing way. Not cheating or being unfaithful, more like: wanting all the affection, wanting all the closeness