The ALEXANDRINE is a line of poetry that has 12 syllables and derives from the French who don’t believe their language is stressed. As such, their poetry is syllabic, and the alexandrine is a twelve-syllable line.

COUPLETS are any two lines working as a unit, whether they comprise a single stanza or are part of a larger stanza. Most couplets rhyme (aa), but they do not have to.

Thus the ALEXANDRINE COUPLET is any 2 rhymed lines of 12 syllables each that work together as a unit...

Example by Jacquii Cooke:

Where Heaven Glows

Throughout the night
The wind doth blow
Heavy secrets of shadows bliss-kissed with romance.
Such a lust binds thy heartstrings in longings of France.
On precipice: light
Where heaven glows.

Throughout the night
The wind doth howl
Lingering moments of a blinded ecstacy
Where memoried pleasures are just so next to me
On precipice: light
Where wolves do yowl.

in the above example lines 3 & 4 and lines 9 & 10 are the alexandrine couplets.
2 rhyming 12-syllabled lines that work as a unit