That looks like something custom made or after market. (It looks like a quality item)
You could block the passenger side outlet with a rag while the engine is running as see what happens.
But... It may not solve your problem with noise as capping one outlet could cause resonance at certain RPMs.
I would give it a go and see what happens. What have you got to loose?
I'm sure the "Flowmaster Dynomax 1 in 2 out" is a good muffler but there is another option.
Go with a Monty...
Monty Mufflers - porsche mufflers stainless steel australia USA - hand, fabricated, heavy, gauge, 304, flanges (no affiliation)
They are a great muffler and it's been around for a while.
They do have a sporty tone so it may not be what you're looking for.
You can order it with either twin outlets or a single... (either 75mm or 100mm)
If it was me I would go with a 75mm single outlet. (but I'm 44 and not into the look at me BS)
Have you considered an OEM muffler?