Creating characters to overshadow the players is a bad idea. The PCs are supposed to be the heroes of the story. At best they should be built on the same rules as the PC’s but should be slightly less.
Being individually more powerful than the PCs may cause issues, but creating great teamwork should be fine. With one NPC being a bloodrager, the other could be built to support that. My guess would be either a controller character like a witch, or a healer type like a life oracle.
I'm rather dubious about the rival (N)PCs being individually that much more powerful than the PCs. Particularly if a guest player gets to use one of them.
Building each pair so that they work well with each other is a good idea. Done well (and with a dash of luck), that can explain why - even with just two party members instead of four - they can complete the side-quests.
And I applaud the idea of having them at all. So many times, a group of players can feel/think that their characters are the only adventurers in the world and that they have all the time in the world to complete quests, etc.
The best way to create a great duo is to build them as a team, not as a pair of high-powered characters. Look for classes with good synergy that make them more efficient when working together instead of being more powerful individually. Two full arcane casters with the same archetype focusing on using weapons have little to no synergy and don’t even work that well.
Blade Adept is a pretty bad archetype and trading away half your feats for one Arcana a small pool and spell strike at 11th level is making it worse. You have two bad saves, low BAB and no class abilities that support each other.
A better choice would be two DEX based inquisitors or Sarenrae taking the Dervish Dance feat and Outflank, Improved Outflank, Paired opportunist, Precise Strike for teamwork feats. Then take Combat Reflexes, Dervish Dance, Improved Critical (Scimitar), Power Attack, and Weapon Finesse for your regular feats. Move into a flanking position (Improved Outflank helps for this.) and you can set off a chain reaction of hits. In Theory you can get AoO equal to the 2 plus the combined DEX bonuses of both characters in addition to your normal attacks. Each attack (including the AoO) takes advantage of the inquisitor's damage boosts like Bane, Greater Bane, Judgement of Destruction, and spells.
The inquisitor is also very good at stealth and deception so avoid a lot of trouble when they need to. They have two good saves and being DEX based boost their reflex saves so they have no real poor save. Get a ring of evasion and once they get Stalwart at 11th level, they take no effect from any spell if they make their save. DEX also boosts their AC, chance to hit and even damage, so they don’t need more than 13 STR.
Also, I'll freely admit that I have a beef with the aasimar species in 3.x and PF. Historically (back in 2e), they had a -4 Con penalty which should have translated to at least a -2 Con penalty in 3.x. Anyone hoping to play an aasimar at my table will likely be disappointed by the house rules nerfs which I've given that species.
Um, I think you have that backwards. They had -2 Con in 2e, which should be -4 in 3.x. Don't get me started on what they did with tieflings.
Yes, you're quite right there about me getting the ability score adjustments mixed up between the two systems. That'll teach me not to post when I'm tired! :)
The inquisitor is one of the better written classes in the game. The base inquisitor is perfectly functional and the only reason you need an archetype is to make it fit a particular concept. If this is going to be used by a new player I would recommend against any archetype. If the player is reading up on what his character can do having to read multiple different things is going to make it harder. Also lacking abilities, they read about in the basic class section might be confusing.
The inquisitor is a fairly complicated class so might not be the best choice for a new player. Giving a new player something fairly simple without a lot of limited resources to manage is often better than using a complex class. The build I gave relies on using setting up fairly complex game mechanics that a new player will not be aware of and may have difficulty pulling off. If you or another player have to run the character it does not really give them a taste of the game, and they are probably not going to be interested in continuing.
The problem is that your reason for wanting the duo kind of conflict. You want a team of two characters that can handle what an entire group does, but you want to use the characters to give a new player a chance to play. The only way a team is going to have any chance of matching a party of 4 is if they are well written and use incredibly good tactics. I don’t see a way of reconciling those two needs.
If these characters are going to be used by novice players do not make them more powerful than the PC’s. This is not fair to anyone involved. It is likely to cause resentment from the regular players as their characters are upstaged. It is also unfair to the new player as they do not get a chance to experience what actual play is like.
For a new player I would recommend a paladin. Paladins are the toughest class in the game but also have decent offensive abilities. Many of their abilities provide continual benefits so there are less things the new player needs to keep track of. They have enough limited resources that it also gives the player experience in managing those without rendering the character useless when they are gone. The only downside is the chance of losing all the abilities if they fall. Even this gives the player experience in roleplaying so is not a complete negative.