Outside mitten:
After putting thumb stitches on waste yarn, knit charted pattern repeat for 5 times starting with round 2 (I finished my thumb increases on round 8 of the pattern and then knitted another round, thus round 1, for putting the thumb sts on waste yarn and CO 2 extra sts)

15 thumb rounds after picking up thumb stitches.
Then decrease.

Inner mitten:

On smaller needles, with outside yarn, pick up and knit 48 sts from the cuff and knit one round. Break yarn, take inner mitten yarn and knit as many rounds till you reach the length of the cuff, just before the start of the thumb increases of the outside mitten.

In my case 17 rounds.
Increase for thumb in the same fashion as the pattern says for the outside mitten (increase 2 every third row) until you have 18 thumb stitches. Make sure you place the thumb at the same side as the outside mitten.

Put 18 thumb stitches on waste yarn, CO 2 (tbl) and knit as many rounds needed so the inner mitten will fit your hand and in the outside mitten.

Pick up 20 thumb stitches and knit as many rounds needed (in my case 15). Decrease in the same fashion as the pattern stated for the outside mitten thumb.

Wet block when inner mitten is inside outside mitten. (Like you would wear them)