It would depend on the spell. However the effect would simply be amplified. If you have ever heard of a Zaizen curse (probably misspelled), if multiple casters take part in the casting the power is amplified from each caster.
For example, if one man casts a hex then it is that one man versus the other, If three casters take part in the hex the power is amplified by the other two and the ability each holds. However then you need to look at the ability of each caster.
The novices may equate to one master so its all dependent, hope this helps.