Kazaan's explanation is almost perfect, though I request one point to be clarified.
On the second round after you have cast frostbite, and are still holding the charge, you can no longer get a free attack from spell combat. However, you could of course make a TWF round of attacks, and attack using your off hand as usual at -4weapon/-8touch, or -2weapon/-2touch if you have TWF the feat.
Could you then accordingly use spell strike for a full round with -4weapon/-8weapon, or -2weapon/-2weapon if you have TWF the feat (and your weapon is a light weapon otherwise -4weapon/-4weapon)?
At this point things are getting weird bc you are being penalized for actually doing less, ie only touching instead of casting and touching;
I think I might rule myself that if you want to use frostbite as if with spell combat over multiple rounds, you need to take TWF, in which case you can use it with spellstrike just like you were casting it.
It also might be fair to claim that it just works, as a Magus is essentially supposed to have TWF when it comes to touching and stabbing. You just obviously can't cast a spell and get spell combat again the same turn.
By RAW I think it is a little murky how that all should work.