I agree with letting two players (who are very consistent about playing and this is the core group) play two characters each. I haven't done this with SR because I run 3-4 players a session anymore (but have GM'ed SR groups as large as 10

), but I used to do it in D&D without too much of a headache. And I basically disagree with the players needing to be skilled SR players. What is needed is a skilled GM. Basically anyone can play a RPG... they simply tell the GM what they want to do and the GM tells them what to roll. They quickly learn what all those little numbers mean on their character sheets. I'd even hazard to say that two characters apiece for newbies will encourage their returning to play. Odds are that neither will ever be bored. If they are true newbies to role playing in general, then start them up with one character apiece (and do an easy run or two) and after a session or two, add the supplemental characters and toughen things up some.
One way to handle it is to get a basic idea of what kind of character they want added to the group and then do the basic designs of it as the GM and have the player flesh it out.
There can be a problem if you pick up other players who want to play the game.
One solution would be to start a different campaign with all new characters. Another would be to require the players to quit playing one of their two PC's whenever the extra people show up. The nice thing about SR (all editions) is that new characters can join in with veteran characters without being too wimpy.
The biggest thing in all of this is to be fair to the players. Be willing to compromise in small areas but refuse to in anything you think will upset game balance. Really, that rule of thumb applies to all of SR, not just multiple characters being played by the same player. Remember,
you are the GM and are in charge of the game. If you let the players get more powerful than you can deal with, it's
your fault. And you can always correct that. Toughen the adventures up. Let the PC's mistakes start catching up with them. They always have mistakes in their pasts. Take away their toys, take away their nuyen, hurt them, force them to go underground. And yes, sometimes characters die.
But always be fair. It's one thing to make adjustments in the game and take away possessions. It's another to be perpetually oppressive and never let them win. The characters should always feel like they have a chance. And if you get the expectation to the point that a "win" is staying alive and "maybe" getting the job done, and maybe scoring a few nuyen to pay the rent and buy the ammo and replace that car they sacrificed, then you have Shadowrun down to it's true core.
GM run PC AssistantsI frequently have NPC's that step in to fill "gaps." The key thing to remember is to play them like an NPC in attitude. They have a motive for doing what they do. If they are helping sort things out along with the PC's, use LOGIC tests to see how well they provide info, etc. And remember most of all, they are a GM tool. Don't be afraid to use them, break them, rip them to shreds. Having an NPC companion who's been around for several runs and has a personality that buys the farm while saving the hoops of the PC's can be almost as sobering and emotionally powerful as if a PC kicks it.
Vlad