Swelter is sorely limited by being two different creatures. If it had the ability to hit a single thing for 4, it'd be a lot better.
Seriously, though - Shock when you absolutely have to, Lightning Surge when you can wait until their end of turn. It's a great idea for a card; but it needs to be a lot higher cost.
Mmm. If this spell said "Deal 2 damage to target creature. Deal 4 damage to up to one target creature." it would cost... I don't know... maybe ? Maybe ? That doesn't even count the versatility of still being able to use the card as a creature-only Shock. Not being able to cast two spells is a drawback, but, with this card, only starts to become a drawback when you have 6 lands in play.
Deal 6 damage, split two ways, for ? Ok, only if it's your only spell at the end of your opponents turn - how often is that gonna be a problem? Seems too good to me.
given the high mana cost, it's unlikely you have mana left for other spells.
Changed the cost and made damage output 5.
Swelter is sorely limited by being two different creatures. If it had the ability to hit a single thing for 4, it'd be a lot better.
Seriously, though - Shock when you absolutely have to, Lightning Surge when you can wait until their end of turn. It's a great idea for a card; but it needs to be a lot higher cost.
Swelter has a community rating of 2.293. Just throwing that out there as a warning.
Given Swelter, jmgariepy's hypothetical card would likely be at least
.
Mmm. If this spell said "Deal 2 damage to target creature. Deal 4 damage to up to one target creature." it would cost... I don't know... maybe
? Maybe 

? That doesn't even count the versatility of still being able to use the card as a creature-only Shock. Not being able to cast two spells is a drawback, but, with this card, only starts to become a drawback when you have 6 lands in play.
Deal 6 damage, split two ways, for
? Ok, only if it's your only spell at the end of your opponents turn - how often is that gonna be a problem? Seems too good to me.
I removed dualcast from it and was going to include a new mechanic. I couldn't come up with one though, so it ended up being left like this.
Heh, in fact, that'd be expensive even at that cost: that's half of Order // Chaos which cost
:)
Mmm, for this cost you could probably print "Creature's cannot block this turn"
Seems rather worse than Nightbird's Clutches.