The PHB, page 88 states about metamagic feats that spontaneous usage of one metamagic feat increases the casting time to a full round action, or an additional full-round action if the original effect takes longer than a standard action to activate.
If a second metamagic feat is used in conjunction with a single spell, does that add extra time beyond the first? Once the first metamagic feat is added, the effect has a longer casting time... so a second metamagic feat should possibly add an additional full-round to the time....
The book does not seem to clearly state either way.