I might not remember all of it, since we've modified them as others have, but... (RMSS)
There's a lot that goes into this. Firstly, you need at least 60% action for a melee attack and you can only parry a foe you have an attack declared on. So you just won't be parrying multiple foes at all unless something else is brought into the picture. Two Weapon Combo, Haste, Adrenal Speed, spells, whatever.
If you're Hasted, using Adrenal Speed, etc. then the rules are the same, you just have 200% to work with so you can make one attack on up to two foes or three attacks on up to three foes and just follow the normal rules. You could potentially make one attack in each Phase, probably something like 70%, 60%, 60% give you an overall -50, -40 and -30 in the respective phases and using the remaining amount to parry.
If you're using two weapons you have to develop the skill for each hand separately. So even if you have two Broadswords you need to develop two Broadsword skills (one for each hand) or you'll suffer the -20 off hand penalty (unless you have Ambidexterity). Lastly, you still can't actually use that second sword unless, again, you're Hasted, Speed, using Two-Weapon Combo, etc. and at that point there's no point to having the second weapon anyhow (except for TWC).
If you're using Two-Weapon Combo splitting your attacks between two foes is allowed, but they must be in your forward arc and you have a -20 penalty for both attacks. Parrying amounts applies to both attacks too, so if only want to parry one foe (in a split attacks situation) your other attack is still penalized by the same amount.
The spell still seems to require you to split up your OB between two foes (so you don't get to use all 100% vs both). It's pretty useful if you're going by the formal rules and you only use a single weapon. If you're using Two-Weapon Combo you're already effectively doing what the spell does. Using the spell you can split a single weapon 100OB into two 50DB parries. With TWC you can take 100OB, -20OB for splitting attacks, and have 40DB parries without the spell.
One of my sticking points with that spell (Split Parry) is that if you have 200% and you attack two foes for 100% each with the same weapon I don't consider you to be splitting your attacks, which draws a -20 penalty. They are two unique/individual and separate, attacks. So the only time I see this spell being particularly useful is if you have a single weapon, have your normal 100% attack, and need to parry two foes, or if you are using TWC and need to possibly parry three... or four? (in which case we'd have to discuss if the instant spell in question would apply to both attacks, which I would say no, or if you can cast more than one instant spell per round, which the rules say no - but we do allow if they are not offensive spells).
Personally I will allow someone to parry anyone in their forward arc if they are a Pure Arms profession, make a Combat Awareness Check, or are parrying 100%, but you still have to split the OB up, so I'd probably rule the spell allows a more effective Parry as a result.