What's your question? Ask

This article will give you some answers: https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm

This article should be of some help: https://www.gardeningknowhow.com/edible/herbs/cilantro/how-to-harvest-cilantro.htm

You don't say whether you're wanting to grow it indoors or outdoors, so here are two articles: https://www.gardeningknowhow.com/edible/herbs/cilantro/how-to-grow-cilantro-indoors.htm
https://www.gardeningknowhow.com/edible/herbs/cilantro/tips-for-growing-cilantro.htm
I have noticed that the potted herbs that some grocery stores sell generally don't do very well compared to potted herbs purchased from a garden store.

This article will help with that: https://www.gardeningknowhow.com/garden-how-to/seeds/storing-seeds.htm

I personally would leave them together, or at the very most cut the pots in half. The way that cilantro grows and how it is harvested, not too much harm will come from them growing crowded. As long as they get enough water, you should still be fine.

Hard to tell what's eating the cilantro with no description of the damage, e.g. round bites out of the leaf edges, or whole plants gone, etc. Cilantro is not much troubled by insect and disease, although slugs might have a munch or two. Deer supposedly don't much care for it, but rabbits, mice, raccoons, and others will eat it. Here's an article about slug control: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm
This article is on rabbit (and other small animal) control: https://www.gardeningknowhow.com/plant-problems/pests/how-to-keep-rabbits-out-of-gardens.htm