It is, but you'll have to spend practically every point you get doing it. Also, it is certainly possible to have both maxed out Frost and Fire. The use would obviously be to freeze everything first, and then use fire once you're in a good position. However, it does mean that you have to spend 180 points on those two alone, which means you'll have to cripple your mana a bit.
I have completed it once with Frost as my main spell (Inquisitor) and once with Fire (Mage), but I've never tried having both.
I have completed it once with Frost as my main spell (Inquisitor) and once with Fire (Mage), but I've never tried having both.