Eh, as much as I want to accommodate I'on being a skilled mage and everything, casting two spells with so many words - several of which are long and/or pretty to pronounce right, and even more with difficult-to-weave runes associated with them - within a period of, what, three seconds or thereabout... it seems inhuman, to say the least, and even more so that neither of them blew up in his face in the attempt. It's quite simply just too much in too little time.
Also, in the magic of The Prophecy, "more words" does not equal "more powerful", or even necessarily "more effective". Power - which is to say the amplitude of the effect conjured - is controlled purely by the amount of energy poured into the spell in arcane magic. What spells with longer incantation gain isn't that, but rather complexity and, in many cases, a sort of expanded "artificial intelligence". More words in a spell supplies the magic with more elaborate instructions, allowing them to manifest as effects much more advanced. Whereas the fireball-spell (Purthev cowath thagir meethal dregoth. "Summon [a] powerful explosive ball [of] fire.") just gives the instructions "be a ball of fire and explode", for instance, bonds of the land (Thoph smaiwe gohn grumert sonedth, tuagar eren meethal caihl menrirl, lemesatheli dweneth lemesatheli, dweneth sonedth meethal xarthliir caihl menrirl, kohrx harteor dweneth, lemesatheli brega, jhoon cowath. "Summon [the] hardness of stone, small [and] flat spheres [with] holes [of] air, copy [and] lock [these together and] copy [again], lock [together] strongly [with a] cylinder penetrated [by] air, alternately open [or] locked, copy all, [put it in] my power.") not only creates a much more complicated form (in this case, shackles) but also supplies it with a set of instructions on how exactly to act after being invoked. So, eh... the point I'm trying to make is that "longer spell = more likely to work" is not always true. Yeah.
Also, just in case this wasn't clear (I think I told Ashgan about it a long time ago, but I'll go again), spells can't be made up on the spot, nor can one just slap an extra word or ten on a spell they know to change it. A spell has a certain incantation, and if that incantation is used, it will be conjured; any variation from that incantation, and it won't work (in fact it is, once again, liable to blow up in your face for trying).