What is an analysis of the third stanza of "The Good-Morrow"?

John Donne's "The Good-Morrow" is a classic love poem, likely written by the poet's wife, Ann Moore. The first two stanzas draw out the poet's love for this woman, proclaiming love itself as the all-important event in life.
The third stanza continues on a similar path, beginning with an image of the two lovers looking at each other proclaiming, "My face in thine eye, thine in mine appears". This line refers to the two lovers gazing closely at each other, seeing themselves in their lover's eyes. It can be considered on a literal or metaphorical level; eyes do offer literal reflections, and one can see oneself in another's eyes, but the metaphorical interpretation suggests that John and his love only see each other because they're so deeply in love.
"Where can we find two better hemispheres/without sharp north, without declining west?" Since a hemisphere is only half of a sphere (think of each hemisphere as being half of a globe), Donne is drawing a traditional yet poetic image of the two lovers only being half of the entire whole. Donne is not complete without his love, and she is not complete without him. 
The poem continues, "Whatever dies, was not mixed equally". Donne is explaining that true love cannot die, but that true love also requires reciprocal effort. Each lover needs to contribute equally, and only if the love is true can it never die.
The poem concludes, "If our two loves be one, or, thou and I/Love so alike, that none do slacken, none can die". With this final line, Donne continues the previous thought of eternal love and mutuality. If he and his lover are able to love each other truly, wholly, and equally, they can transcend mortal death. Their love is more powerful than all, and it cannot simply die. It's also interesting to note Donne's usage of the word "die" as the final word of the poem. In a literal sense, the poem ends at the word "die", just like a life is ended at death. Yet, the poem transcends time and escapes its death by existing as a form of love and art. Like the love described in the poem itself, "The Good-Morrow" is eternal.