Oh? I thought one of 'em was doing quite well in a briefcase.
If anything, being in a host body handicaps these critters, so that they no longer can communicate with each other, and may be fooled in various ways because of that.
Host bodies may have multiple uses, though. With one critter buried inside the neck to control the host, further ones may choose to live in the stomach. Which is why the hosts start eating copious amounts of worms, and why a creature can leave a victim through his mouth without shedding blood... Perhaps at least one of these tummy tenants then grows bigger, for whatever reason, and ultimately consumes the host. (In which case Remmick would have been possessed first, and was near the putative terminal stage in the episode, whilst his boss did not yet have irreversible body cavity damage.)
Timo Saloniemi