Originally Posted by Pyromancer999
That IS a concern. And I think I know of a way to discourage the Nova Method: How about they have to divide their CL amongst the spells? I'm also open to other alternatives.
Ooh, that's an interesting idea. Hmm, I'd need to do a couple of mock-ups with casting 9 Magic Missiles each at CL 2, versus fewer at a higher... I'll get back to you on this.
After contemplation: Before you go too far into mitigating factors, consider how exactly you want this power to be used. Do you want it for the occasional dual spell? Do you want players pumping out large numbers of low level spells? Do you want this to be used often, or infrequently? These matters should be determined before you decide how to make it a bit less powerful as how you go about that will affect how the ability is used. The CL dividing idea is indeed valid, but means that most damage dealing spells wouldn't be worth Weaving as you'd end up with more targets but a lower total damage. Out of combat you'd have the time to cast spells on separate rounds for larger durations or greater effect. So the only time I'd see this being used with the split CL is when you absolutely have to cast a utilitarian spell as well as another, as when your ally is at -9 but there are a swarm of Tucker's Kobolds who are about to do the same to you. This would allow you to save both your hides, though with a lessened effect to each.
Limit the number of additional spells allowed. This stops mass 1st level spell swarms, while not hindering dual-casting. Still a bit of a nova, just not near as bad.
As previously suggested, add an additional cost of some sort. Either an amount based upon the number of additional spells, or the level of them are both viable. Or perhaps doubling the cost entirely... Any of these would result in it being an option to mass-swarm or use other nova tactics, it just means that you'll get less bang for your buck and be useless the rest of the day.
Require some manner of Spellcraft Check to successfully link spells, with the DC increasing for more or more powerful spells. Failure indicates that the whole lot of them fail, yet the power is still expended. Adds the potential for some dramatic situations, allows for routine use of the ability, but dissuades players from gambling large numbers of levels each round.
Add limitations regarding the type of spells. Perhaps you can't cast the same spell more than once in a round. Perhaps you can't cast spells with opposing energy or alignment descriptors. Perhaps Summoning spells can't be Woven due to the amount of energy used to cast them (and hence the longer casting time). Any of these would add something interesting for players to have to work around. The first suggestion here stops some of the most obvious ploys.