For two people, the key is depth of field: at f/1.8 it’s often too shallow unless both faces are exactly the same distance from the camera. The usual solution is to stop down a bit to increase depth of field, then keep the background far behind them so it still blurs nicely.

Best practices:

• Place both subjects on the same plane whenever possible.
• Move them farther from the background.
• Use a smaller aperture than f/1.8 if needed.
• Check depth of field with your camera’s depth-of-field preview button or a DOF calculator.

For focus, focus on one person in One Shot AF, then fine-tune manually if needed so the focus plane covers both faces as evenly as possible.

If the subjects are at noticeably different distances and you need both perfectly sharp, one workaround is to shoot multiple frames focused at different points and blend them later in post-processing.

Relying on multiple auto-selected focus points is less predictable than controlling focus and aperture yourself.