Betwixt mine eye and heart a league is took,
And each doth good turns now unto the other:
When that mine eye is famish'd for a look,
Or heart in love with sighs himself doth smother,
In the first  quatrain  the Bard’s eye and heart have reached an arrangement to spare him pain of being away from his love. They are unified to keep this sense of love strong for the beautiful boy. 
 A league is a pact entered into by two or more parties for protection or defense.  The Bard’s heart and eyes are  personified , as if entities separate from himself. Therefore they may be working against his better judgement or will.
Sonnet 47