It helps to know the approx area of the space.  Obviously, a bigger space would see more benefit from a 2nd sub.  It also highly depends on how important bass is to you.  I’m personally not that in to bass, so a single sub works great for me.

There is also such a thing bass nulls, which I am not an expert at, where 2 subs can cancel each other out and you get less than a single sub itself.  That’s dependent on where the subs are located.  Others can explain that better than me.

Lastly, consider upgrade path.  For the Arc, one of your 2 subs must be a Gen 3 or Gen 4, so you should be fine there if you add a new sub.  For Arc Ultra, you can use any gen for a single sub, but 2 subs requires both subs to be Gen 3 or 4.  In other words, if your existing sub is Gen 1 or Gen 2, and you want to upgrade to Arc Ultra some day, you won’t be able to use your existing sub in a dual sub configuration.  A little complex, but something to factor in with your plans.

Thanks for your comment melvimbe - to add some requested context. The room is about 12x20 ft and I current have an Arc, 2 Era 300s, and a Sub 3 (considering adding Sub 4).

I’d lean to one in each front corner as a first try.

Get a couple long extension cords and try moving them around the room to find the best overall sound.

Don’t rule out anything, even under the couch can be a good option.

If the appearance is an issue you can buy or make fabric covers for the subs to help them blend in. The covers are also excellent critter and grandkid proofing.

Once you are fairly happy run Trueplay if you can and fine tune things.