For resistance style attacks, I'd go for a RR penalty, not allow the forcing of two or more RRs.
For elemental attacks, I might allow two bolts ala "Fork of fire" on one target, but like other multi bolt attacks, only one would get directed spell bonus, the other would be +0 OB.
If using a bolt spell, it would then seem a better idea to SM x2 damage than to attempt to hit someone with two bolts, unless their DB was so high you're just "spraying and praying" for a high open ended hit.
But I also don't allow SM spells to dupe higher level spells on the same list, so if there's a multi bolt spell at a higher level, I wouldn't allow you to SM a lower level spell into a duplicate of it.