I use greenstuff for soft stowage, glueing things together, reinforcing and covering joints.

I use Milliput for bases, smooth joins, filling holes in plate structures (holes in armour) and where a smooth surface is required. If you look on the Sherman above you can see here I have used Milliput around the pistol port.
The ability to sand and drill Milliput is really useful.
Coincidentally, I am using both on my Panzer Ib.

The next stage is to fill the gaps on the superstructure body join with Milliput and blend in the greenstuff.
In one of the old SF&Fgaming magazines(Harbinger?), I think it was an ex GW person called Tammy recommended using a Milliput slurry to smooth the surface of white metal where there was surface pitting (some figures can have less than optimum surface smoothness). It would also work on 3D prints and where you cannot reach with sandpaper.
You can also put surface texture on surfaces that are meant to be cast, such as Sherman turrets.
I find greenstuff is quicker and cleaner to use but Milliput still has its uses.
I have never tried mixing them.