Is it possible to put multiple textures on one object, for example different faces of a cube, without separating them?
Yes, both in BI as in Cycles.
which render you choose will determine a different wokflow, but basically, you need to define your masks and use them to mix textures.
for example, in cycles, you can use the MixRGB to mix two textures, based on a mask (where black will show the top texture and white the bottom one).
those textures can also be results from other MixRGBs outputs, so you can add and mix even more textures.
For textures, so long as you have your object UV Mapped, it’s not a problem. If the textures are not overlapping on the same space, you may even get away with simply creating a couple of materials and assigning them to specific faces in edit mode.
Yea You can do as Colkai say…You can also assign image to back side of faces…
If You just use the U - Unwrap in Edit mode…If a Cube it will just stack all face on each other.
You select a face in edit mode and add a Materiale and assign it it to the face…Now put a Texture on it…E.C.T
You can also use U - Smart UV Procject in Edit mode…It will will unwrap the face to the texture space.
For back side…In Node Editor add a Mix shader copy diffuse and texture node select another texture plug it in Mix shader.
add…Input - Geometry Node plug Backfacing into Fac of Mix shader…