Multiple exposure is not the right tool here. What you want is slow-sync flash (also called dragging the shutter).

Use one normal exposure with flash and a longer shutter speed. The flash fires for a very short burst, which freezes and lights your subject, while the shutter staying open longer records the darker background and city lights.

Typical approach:

• use flash with TTL if available
• choose a slower shutter speed to bring in the background
• hold the camera steady, ideally on a tripod or other support
• ask the subject to stay still to avoid ghosting

If the subject moves while the shutter remains open, you may see blur or a faint second image from ambient light. A steadier camera, less subject movement, or a slightly shorter shutter speed can help.

So: don’t use the camera’s multiple-exposure mode for this. Use flash with a slow shutter speed instead.