Probably not enough weight with the "spare spare" to make any towing differences so it seems OK to mount them side by side.
But what makes you think you need two (2) spare tires? I'm betting you have very common size tires on your trailer so even in the event of a complete blowout a replacement tire sill be easily purchased.
I've used my spare tire one time. Old tire blew (all of them since replaced) -- felt like I'd bee rear ended -- and the spare was actually older!
But I didn't need a second spare.