Well, it is not that simple that 'it is a pro version'. #21 is a very advanced product with a unique appearance. It looks balanced, wonderfully crystal clear and clean with great depth and wetness. One of the products that can be identified easily and is in fact distinguishable. NXT (paste!) is almost as durable for me, but looks definitely different. It looks clear and reflective, but without the characteristic silvery cleanliness of #21. Their cleaners are very mild so two (even three) layers will always look better than one alone. 2hotford (Megs Online mod) uses a #21 ×2/NXT paste ×2 combo and it looks insane. I tried them both ways (#21/NXT vs. NXT/#21), and I must say I really like them!
So, I'd apply 3 thin, gentle layers of #21.