I carried both my DDs for a while - felt a bit like a pack horse but it was actually more convenient and comfortable than it probably looked.
I had
DD#1 in an ergo on my back and
DD#2 in a ring sling. It took a bit of jiggling, mainly to get
DD#2 sitting around the straps to the ergo, but it was pretty good. You couldn't use the ergo on your back for a newborn but as the PP said, maybe a mei-tai could double. Not sure I would want two ring slings as I find that they put a bit too much pressure on my neck and shoulders.