To make a mixed race character, I followed the following steps.
1. ~ showracemenu
2. Decide the primary race, and select it (don't finish the race menu yet)
3. While still in the race menu, ~ setrace XXX (setrace woodelf, for example)
4. From here, your character will take parts from both races, and you can edit your character until you're done.
5. PROFIT!!!
This is a picture of my Redgaurd/Orc, Janessa.
http://cloud.steampowered.com/ugc/631859135471553891/755CB87858E471FFDB7E0A53A959F9EFFE596BDE/
Ugly [censored] Nord/Orc, Uggmugg
http://cloud.steampowered.com/ugc/631859135472788809/FC6B419A2B344DE3FDFDC64D5412FB09DD19C114/
Dunmer/Altmer dude
http://cloud.steampowered.com/ugc/631859135473526619/466198D98FEF497FA26CAF8490101619E7A8E1DB/