Title: THE MARRIAGE PROJECT (Make-Believe Matrimony, 25) previously published as the Marriage Assignment
Description: They were assigned to be marriage partners

Gillian Flannery's marriage project worked great with her high-school students. But when she was assigned a partner herself, her "marriage" to John Slater quickly ran into trouble. 

In real life, Gillian soon learned, people came burdened with troublesome personal baggage. Gillian herself had a deep aversion to conflict. As for John, he came with two half-grown daughters who bickered constantly. Worse, his ex-wife had just breezed back into Indianapolis and declared war.
Did Gillian ant to get caught in the cross fire? Not likely. But for John Slater, she just might dig in and fight.