Here's how you might make a see-through fireplace into two back-to-back Rumfords. Construct a common fireback and covings and replace the damper with two smaller dampers.

We don't generally recommend venting two fireplaces into a common flue but there is nothing in the code that prevents it. It might be necessary to keep the unused fireplace damper closed in order to get enough draft in the other fireplace. When using both fireplaces, get one going before trying to light the second one. And don't ever convert just one to gas. It's dangerous to vent appliances with two different fuels into one flue.