A new type of uv map is planned for blender 2.8 that lets you divide the map across different image files, it looks very interesting. Normally this would be off topic, but this would be very useful to incorporate in make human, solving the long time problem of making higher res facial textures.
https://code.blender.org/2018/06/cycles ... ode-quest/
edit: technical info on udim: https://www.fxguide.com/featured/udim-uv-mapping/