After their identities are stolen, a young couple need to find their enemies, fast.

Why must they discover who stole their identities? The answer is; identity fraud, best you describe that as the inciting incident, not the house swap.

After their identities were stolen, a couple must...

Inciting incident aside, you don't describe their goal, which makes the plot vague. If their identities were stolen their goal is to stop the thieves not just find out who they are, this may seem implicit, but in loglines you have to describe the goal and how they plan on achieving it.

"When their identities are stolen, a couple on the verge of buying their dream house must..."

Okay, who the hell swaps houses?

How about "After a real estate purchase,...."