A little trick for Milliput Bill, preferably the grey/yellow version, not the white (the latter for repairing crockery etc.)
Cut equal, small amounts off each 'stick'. Roll these into individual 'snakes', keeping them pliable and warm as you knead them. Once each 'snake' is soft and pliable, plait both 'snakes' together, then roll into a ball, mixing the colours together. Keep kneading, rolling, pressing, rolling etc, and the colours should blend and the material become softer. Then it's ready to use.
However, depending on ambient temperature, and the amount being used, it can take quite some time (in hours) to set hard.
I've only ever used the white version once, and found it seemed to take forever to harden, compared to the grey/yellow.
Hope this helps.