And they must have thought they dreamed

Couples.

or

Maths for beginners.

.

B+A= a couple of newlyweds looking ecstatically happy most of the time.

.

A+A=two colleagues having a smoke and a drink.

.

A+B= couple of newly divorced looking for a new BMW.

.

B+A= half of ABBA having fun on stage

.

A+B= other half of ABBA go for the more serious stuff