Making a one sided fireplace two sided is possible. If you are considering this project then there are some important things to consider.
What to Consider When Making a Fireplace Two Sided
You will generally want to make this conversion only if the fireplace is already a ventless gas fireplace or you intend to make it a ventless gas fireplace. When dealing with a vented fireplace, the flue will not be large enough to handle the double sided fireplace. You can use a fireplace exhaust fan but these are quite expensive.
Before removing the rear of the fireplace it is important to determine if that portion of the wall is a load bearing wall. If the portion of the wall you will be cutting out is supporting a large portion of the chimney, then it may become structurally unsound if you remove it. This is often not a problem since many homes are not built with a true chimney.