Texture paint mode: is is possible to paint over multiple materials? Let's say I have a cube with two materials assigned to it: cat and dog.
The cat material is assigned to one face, and it's a simple image texture node with a picture of a cat. 
The dog material is assigned to another face, and same node setup.

Now I understand that a model can be assigned multiple materials,
but each face can only be assigned one material, and so my question is:

Is is possible, in the texture paint mode specifically, to create a painted line that would start from the dog face and end in the cat face?
If so how?

Now it seems to me that it's not possible unless you make two textures (splitting your painting in two in this case), one for the dog, and one for the cat, but I'm a beginner so I still hope it's possible somehow in a simpler manner...
If you are using Cycles, this is as easy as copying and pasting the new overlay image texture set up into the second material so that when painting on the single image, it is references in both materials and can allow interaction in Texture Paint mode with Material Draw mode. I have a screen shot to show a simple two side set up.