Call 2-3 guards to the exact same area with your bells.

Use Kurosuna (preferably Kurosuna II) on all enemies you attracted to the spot with your bells. This way even if you walk in front of them, they will not see you therefore, you'll still be performing a stealth kill.

Walk up and make sure you're hitting your space bar quickly. As soon as the first guard's stealth kill animation ends, you want to be spamming your space bar for the next stealth kill. This all has to be done very quickly.