My thoughts on two 10s vs. one 20 f I may as you try them on...
Not sure if you were going to double it to make a 20...
While two 10s seems so "versatile," in my opinion, we pay big prices for these pieces to look great on us and fit properly. Jewelry is not one size fits all, and short necklaces need the most adjustment to land correctly.
That said, as you try on the necklaces, try on the 10 as if you were going to wear it alone. Ask yourself is it the correct length, does it need shortening/lengthening, and will it lay correctly with the tops you wear. I think the most important thing in wearing a 10 is the length. Often times I see a 10 land in no man's land or it competes with a neckline of a blouse, so pay attention to that.
Take that 10 (shortened or lengthened), add another 10 and see if you like the way it looks. Does it land correctly, and will the clasp bother you? You will definitely see clasps with two 10s together, no way around hiding it. I'm a purist when it comes to jewelry, and I find clasps showing very unattractive, especially for fine jewelry, cheapens the look in my opinion. For some people, it's no big deal, so think if that matters to you.
Try on the 20. Look at how it lands on you and the overall look. You will not see any clasps. All you will see is one continuous beautiful strand of motifs. You can probably guess by now I love 20s. They work with all necklines, and add such a pop to any outfit.
Plus, since you a big bracelet person, I think a long necklace bridges your pieces beautifully! The eye first sees that striking long necklace. Honestly, you cannot miss a 20! It's nice and long, and the eye moves down to the torso area. Then, the eye thinks.. there must be more of this beautiful stuff, and from the torso, the eye gently looks horizontal to the bracelets on your arm. Very flowing and so beautiful.
If you love short necklaces, I think one 10 and one 20 worn together look fabulous. I love the look and have actually thought about getting a 10 also. But I know myself, I am not a short necklace person and I will never wear it. I think it's beautiful though, especially with the 10 worn short, choker style layered with the long 20. So beautiful. The look is not the same though if the 10 is too long.
I just wanted to share my thoughts to help you decide.
My point is don't just wear the two 10s and say, it looks the same as one 20. Wear each piece separately to see what you would really do if you purchased two 10s. If you just end up wearing it long, one 20 is by far more elegant looking. You can always get one 10 later and wear them together.
Just my two cents. Hope that helps a bit.