No. You can only have one cog on a single speed wheel at a time, per side.

Google single speed bikes and look at some YouTube vids - you don't seem clear on the concept yet.

As per an earlier suggestion, ride in one gear on your geared bike for a while about 70 inches.

With your back issues, I'd suggest keeping away from hills if you do that and I'd check with your dr. first. Like you, though, he's going to need a basic understanding of single speed bikes and gear ratios to advise you.