You actually have answered your own question. You should not be able to obtain a second mortgage unless all parties in title are in agreement. Also, I would advise against a second mortgage. I have seen too many people who decide to sell and move within a short time of getting the second mortgage, then when they go to sell they find they won't have the money to make the move.

I'll reiterate that, no, you cannot take out a second mortgage without the consent of any other owners on title. However, while taking out a second mortgage isn't the best strategy for everyone, the time has come and gone where you can take so much equity from the property that you couldn't sell it later, if needed; whether it's a good idea for you, or not, should be determined after you've discussed it with the other owner(s) and a loan officer who has examined your situation.