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Common mint is not poisonous to cats and dogs, but there may be other plants in the mint family that are poisonous to pets. For example, pennyroyal is a toxic plant in the mint family.

It looks like your plants are in a container too large for them. A small container of soil is best for such small plants. Too-large containers are prone to holding onto too much water, which can damage the plants' roots- this may be the cause of your problem.

Try gently digging up each plant with its roots, and repotting the plants in a small container with good drainage. In the new container, check the soil moisture at the surface and water when the surface is dry.

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There is no problem planting both of your Mint plants in the same container. Once established they can grow quickly, but you can easily keep them trimmed back to keep them tidy.
It is when they are planted in the ground that they become invasive. The roots will send out runners and Mint will pop up everywhere!
If you use containers outdoors for your Mint, lift the pot every so often to clip off any roots that may be extending out of the drainage holes.

Enjoy your new planter, isn't Chocolate Mint the best?

Your plant needs to have a well draining container, if the water stands in the pot, it eventually will kill it due to root rot.
Your plant also needs a full sun spot to do well indoors. A south facing window is good, or you could choose to add a grow light.
Here is a link with more information.

https://www.gardeningknowhow.com/edible/herbs/mint/growing-mint-indoors.htm