I think that again like most everything it depends.
I feel that intention and the materials used have a factor in determining if the spellwork will combat each other or not. Why are you casting more then one spell for the same thing at a time? What some will do is cast a spell and give it a moon cycle to work and if it hasn't or if it hasn't been as strong they will go over what has happened during that moon cycle, they will see their thoughts, their actions etc... and then determine what course to take, whether it be to refresh the spell or to cast a new spell at that time.
If you are casting a spell for money and dont think it is going to work and so decide to cast another to help it out, chances our yes those energies will combat each other because you don't have enough faith and energy/umph into your first casting to carry it out, and that leaks into your second casting. Another thing to consider is the tools and energies used in each spell, some tools and energies do not mix well with each other and some mixed can neutralize each other so thats another thing to consider.
Just my two cents