After doing more research and seeing the various rules in the system. I think that the originator of this took from several rules, maybe some outside of RM, and determined that x2, x3, x4, etc... casting was within the rules, so long as you took into account the rules regarding Conflicting Spells, ESF, Compounded Level with PP, Spell Mastery, the Multi-Attacker rule, and a few others. I think if you look at the action economy of how much % action it takes to prep and cast spells it also works out.
Not really a house rule but a rule that was just spread out among several other rules that he just took parts of to build a unifying rule.
The rule that came out doesn't impact the game negatively because of all the cost and ESF modifiers. Besides if you were to look at the spells most often affected there are spells in the same lists at a higher level do the same, if not greater, thing at less cost in PP and ESF and have greater results.
In the end for the casting of Shock Bolt, it was like casting Magic Missile in D&D (which I think was the catalyst for this), where you cast multiple Missiles based on level for increasing effect. In that regard it was fine. As for other spells that were doubled, the cost to effect ratio was so much greater than it really only helped for spells of level 4 or less anyway. Beyond that, the cost and compounding ESF made it not worth it, while better spells were higher or equal level to the one they were attempting.
Sorry, off-topic:
This was in another post and I have used it myself. Since it takes 90% action to prep a spell and 10% to hold the spell. There was nothing in the rules preventing the caster from prepping a second spell right behind the first and casting both in the same round or back-to-back rounds. This was done for spells that took several rounds to cast and the caster had the time to do it before a battle. Having two, three-round spells waiting for back-to-back casting was within the rules and a viable method. I used to allow it provided SD rolls were made to maintain the first spell while the second was prepped, with dire consequences when it failed.
Sorry off-topic, for another time. But it seemed to also apply in a backhand way.