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