Let me say two things first...
1. I don't believe a DM can improve a player; however a DM can help a player to improve. It's a narrow difference, but it's an important one - if the player resists improving it will be a lost cause.
2. Are these players having fun, and are you having fun with them as a DM? If so, these players do not need "improvement" to an artificial standard. More than anything, D&D is a game that is supposed to be fun. If you're already having fun, everyone wins.
That being said, there are ways that you can help a player improve. Let's look at these players separately...
Player #1, from your description, seems to be a good player, if a little withdrawn. If your player (and the other players in the group) seem fine with it, I'd leave this one well enough alone. If, however, this player brings down the game dynamic with a refusal to speak, I suggest shining the spotlight directly on that player from time to time. NPC's could develop interest in that particular player's PC (romantic, personal, otherwise). Responsibility could be thrust upon that PC. The key is to get that player to dig in and roleplay their way through some situations, preferably situations that the other players cannot or will not take the lead in.
Player #2, on the other hand, may require a little more. The best way to get a powergamer (or anyone who is more concerned with the numbers of the game) to create a good background is to offer something number-related in return. Have the player make a wish list when creating a character, and then make that player justify via background why the character has this item, or this feature (like more starting money to start with, for example). Make sure that you state up front that you have final say on how much history is good enough to qualify for a bonus item/feature, so you can veto anything you might need to.
Here's a new campaign idea that would fit the bill for either player, as long as the rest of the group is game to try it:
The hub character (and I use that term to mean the central character in the party, not the most important) is the recent descendent of a great hero or other well known figure of recent history. Have the player you want to grow as a roleplayer create the backstory of the hero, as well as the method on how his current character became an adventurer. Make him fill in the blanks on what happened to his family line in the interim. The other PC's then play support roles (to begin with), either behaving as protectors with more experience in the world, or as novices looking to someone descended from a great hero to lead them.
In this situation, you've gotten a PC to create a 3 dimensional backstory (one that even a dyed in the wool number cruncher will want to hold onto), and you've made the hub player have to roleplay in a central position, basically putting them on the spot for some time to come.
I'll warn you, don't spring this one on unsuspecting players - make sure that the group would have fun with it first. But I've found this to be one of the more rewarding roleplaying setups in the entire time I've played D&D.