I tried layering when I first started diving, using a ScubaPro wetsuit and Lavacore layers. I didn't like it. I'd rather have one layer of suitable thickness than multiple layers. I think multiple layers for equivalent warmth is simply not as flexible or comfy.
Regardless....
I have a Bare Exowear skin, and a Waterproof Neoskin, and a ScubaPro Pyroflex.
The SP is old. It works, but I just use it for teaching in the pool.
I had the Bare and eventually wanted something different. I used the Bare for a week of liveaboard diving and learned that it's a pain to don when wet. When that fleece interior is damp, or my skin is damp - either way - the fleece does not slide nicely over my skin. With doing 5 dives a day, it was always damp and putting it on and off so much was just annoying.
I got the Waterproof Neoskin and like it much better. Once they are on, I'd say it's six of one, half a dozen of the other. But, the Neoskin seems a fair bit easier to put on, even when me or the suit is damp.
If I were limited to diving in a wetsuit and needed to be warmer than what my wetsuit was giving me, I'd put on a hooded vest over the wetsuit, rather than try to wear a skin under it. I have a Bare Ultrawarmth 5/3 hooded vest and it is great for that. I bought it one size bigger than I would normally wear, so it fits very comfortably over my 5mm wetsuit.
The skins that I have are for diving in warm water. I don't use them as layers with anything else.