Liquify multiple layers

So, I use Photoshop to paint sometimes, and I'm relatively new to the program. Let's say I have my lineart (inks) and the paint/colors itself in separate layers. When I'm painting characters, I sometimes need to use liquify to fix some of their features, like a slightly tilted or elongated muzzle, etc. The problem is that if I select multiple layers, and try to use liquify, it simply gets grayed out.

Using liquify in one layer at a time is not an option because it would never yield the same results in both layers and it would get pretty inconvenient really quickly. Like i can't believe liquifying multiple layers at once is not possible, how hard can it be to implement? Other apps already do this. (On the topic of liquifying... how do I toggle the preview on and off? So that I can see the before and after. I'm using CC 2014 and it doesn't seem to be an option? Unless there's a workaround or keyboard shortcut).

Also, using smart objects feels really overkill for just doing some painting. If I really do want to liquify multiple layers at the same time, "without" merging them, using smart objects seem like the only option. So I have to select both layers and turn it into a smart object, in order to liquify them. And it doesn't let me preserve the layers if I want to rasterize it again to keep painting on separate layers anyways, so using smart objects is effectively useless for my needs. I do not want my lineart to be ultimately merged with the paint. And I'm surely not going to a separate file to edit the contents, save it, and then come back to my main file. Not only is it inconvenient but it also disrupts my workflow. I need to see those layers in the context of the rest of my layers, not in an isolated instance.