I
can, and historically
have focused on a single character at the expense of all others. Shali's draconic shenanigans made her a lot more active than Ollie, Siril, Silia, Mainda, Hugishnak, Vichan, Fon, Mard, Kana, and Cald right up until I created Ilhara. Sure, Mainda got a little bit of scamming done, but between the disabling of the disguise system and the difficulty of getting in touch with the Bladestone Foundation, she was a non-starter. Kana's quest to become a knight made her an obvious outlier among the PCs I just listed, but eventually she was tapped out when the ORH died down.
When I made Ilhara, her activities in the UD became an overriding priority until Shali fell in love with Feylar. After that, it was usually a race to see which PC's friends/interests would come online first - somehow, even adding Amaetha to the mix didn't keep me from maintaining a comfortable balance between the three. In the time before I created Vilmar, Ilhara witnessed the disintegration of the Circle of Song, Shali and Fey came to terms with their feelings for each other, and Amaetha managed to become a citizen of Doron Amar.
Vilmar shattered the balance again (also helped by Feylar steadily dropping off the map), cheerfully pursuing his friendships with Oskod, Luniel and Emma (and occasionally just tagging along with others so he could keep up with their faster leveling rate) until Oskod and Emma were retired, and he drifted apart from Luniel. In that month or so, all other characters were put on the backburner until I started frantically leveling Lamir in anticipation of an early confrontation with some of the Sword Coast's monks (like Emma) or anyone else who figured out that there was only more evil underneath the monk's initial standoffishness and hostility. Even then, Vil's interests took precedence, and when Renor came back, Ilhara's occasional interactions with him took more time out of Lamir - but not Vilmar.
Jando was initially a dud (like so many of my pre-Ilhara PCs), so when Shali failed to make a comeback due to being too tightly bound to the now-inactive Feylar, Renor disappeared, and Mainda's fledgling friendship with Arorn and Lyra was slowly broken up by timezones and level ranges, I made Caili. She just about monopolized December and early January, trying to dodge Rakshee's countless alter-egos as she settled into the adventuring life, tried to learn how to swim from three different instructors (all of which have abandoned her to her fate

), and poured her heart and soul into dragging Elsinan away from his WDD*-mandated path towards evil. This was only occasionally punctuated by Kana finally working her way into the now-resurgent ORH (and subsequently adventuring with her fellow squires, because Aaron seems to be the only full knight they have left

) and Vilmar being commanded to investigate Amora's heritage.
Now? Caili's still working on the swimming and the questionably-effective dragonsitting (when she's not having nightmares about becoming an icicle

), Mainda has found herself some dumb muscle to skew the risk-vs-reward ratio of adventuring back to something she's comfortable with, Kana's doing squire stuff, Vilmar is digging through books, and Jando got a second wind in the form of becoming Xyri's meat shield. (A task he's quite happy with, walking stereotype that he was born as.

) Amaetha's intended return to Cormyr was interrupted by an assignment she couldn't walk away from**, and Shali's likely to come out of mothballs soon, with or without her boyfriend's approval.***
So far, the scheduling conflict between all these PCs has not been as difficult to resolve as you might think. Aside from the fact that Jando desperately needs to keep up with Xyri until the high teens or so, these are all slow-moving plots. As long as I don't skew things too much in favor of any one PC, and as long as I keep track of any milestones/events I really want to catch (for instance, Caili was going to offer to watch over Elsinan as he took his last Frostmage level, but I'm 80% sure I was asleep when that happened, and I'm 99% sure he'd have refused anyway because he's paranoid), all of these are well within my ability to manage. The players involved just don't play concurrently enough for there to be any consistent conflicts, and even when they do... they also play regularly enough that I can afford to skip a session with them once in a while.
(On another note, I'm starting to realize why my DMing record lately has been so underwhelming. The minute I had any free time to spare, I threw it at half a dozen PCs, simultaneously draining any resources I might have to DM with
and causing playerside involvement with just about anyone - player or PC alike - I might have thrown random events at.

)
* - White Dragon Disciple. They're all Chaotic Evil, and by this point, everyone on the Sword Coast except Caili is willing to accept that Elsinan's a ticking, everfrosty timebomb and wait for the day when they'll have to rain holy death down upon him.
** - Seriously, if I thought anyone cared enough to do that, I'd say the DA folks pulled out their chat logs of previous interactions with her and custom-wrote a plot to push all her buttons at once. With outside assistance, no less! 
*** - Not that I expect she'd meet with any serious disapproval on Fey's part. It's more that they've reached a stage when it's difficult for me to independently estimate what's going on offscreen to a sufficient degree that I'd be comfortable sending her back into the world on the basis of those estimates. It's bad enough that she'd have to keep her mouth shut about where she'd been these past months, but when I don't even know if she'd want to go out without him at any given moment...
I so need to talk to Smirk about that. Either drag him back in, or work out some kind of offscreen routine I could bend her activities around.