I'm a level 7 mind blade who just got the dual weapons ability:
When using two-weapon fighting with two psychic weapons or a psychic double weapon, the mindblade can use her spell combat ability as though she had a hand free.
Can I use two weapon fighting at the same time as spell combat? How many attacks can I make?
1 main hand, 1 offhand, 1 spell combat delivered through main hand
or
1 main hand, 1 spell combat
Do I take -4 to all my attacks for using both at the same time (supposing my offhand is light)?
You can use TWF with Spell Combat. That lets you make two attacks at a -4 penalty (assuming the light weapon) and also cast one spell. It doesn't specify which hand casts for Spell Combat, and I'm not sure when that would matter. If you are using Spellstrike to deliver a touch spell, it can be with either hand.
You can two-weapon fight as normal, so that's one main hand attack, one off-hand attack, and one spell, so possibly a Spellstrike from your main or off hand.
As for the penalty...the most straightforward reading says -4 for your regular twf attacks, -2 for your spellstrike attack. That's -2 for spellcombat and another -2 from TWF that only applies to the actual twf attacks. I think the intent is -2 for all the attacks, treating the -2 from spell combat as a TWF penalty.
That's great, thanks!