Bridget & Siobhan are both Buffy.
Buffy gets hit with the Doppelganger ray that split Xander. The post-death icy distanced cranky side of her becomes Siobhan, the Fun side of her becomes Bridget. Siobhan goes to work for Wolfram and Hart and becomes fabulously wealthy. Bridget teams up with Richard from Lost to investigate the Hell Mouth, which consumed Sunnydale, and spout it back up as a mysterious Island where 1-2 years later a plane crashes, leading to Lost. They locate another Hellmouth in Wisconsin, but Bridget becomes suspicious of "Victor"'s interest, and after finding out he's not a Marshall with Witsec, and finding an ID revealing his name to be not what he told her, but Richard Alpert, she knocks him out and ditches him. She refinds her other half, just in time for Wolfram&Hart to have Siobhan-Buffy fake her death, and compromise Bridget-Buffy.
"And that is the story of Ringer. At least in my mind."
Bridget is the Buffybot, Siobhan is Buffy
We know the Buffybots are based on sex toys. It makes perfect sense that a Buffybot would eventually settle into a job that would fit with it's primary objective of being a sex object. Not knowing she's a robot and powerful fighter she's unaware of just how little danger she's in which explains her actions. Buffy never did like the Buffy bot however and if she could get rid of it she would. It's not difficult to see a Buffy who could take time off from her job fall in love with an older man who could take care of her financial problems.
Gemma arranged the hit on Bridget.
The night Bridget was attaked by the hitman, Gemma called her and said she knew her husband had been having an affair with and asked her to come to the loft. It seems like a pretty big coincidence that she asked her to the loft at that time of night, just in time for her to run afoul of a hitman. Her goal was to kill Siobhan (or rather, the person she thought was Siobhan) for her role in her husband's affair.
Gemma faked her death to frame Henry and look for Siobhan
Bridget is pregnant too, and the ultrasound will reveal it
- It seems like it'd be quite a surprise and twist.
Siobhan planted agents in NA meetings all over New York City.
Just in case Bridget showed up at one, over a few days every single NA meeting in NYC—except Staten Island—had a spy she hired to keep an eye on Bridget. Then Bridget emptied her slush fund....
Siobhan is being manipulated by someone we haven't seen.
Think about this. Considering how well her plan works out (which is VERY VERY WELL and she nearly suceeded if not for a last second hitch) it was amongst the worst plans possible. First Bridget and Siobhan haven't spoken in years at the start of the story and I'm sure the FBI didn't reveal the name or identification of the high profile witness they had "lost" since that would pretty much paint a target on her back. So how did Siobhan even know that Bottoway was hunting her sister? Because she was told. The person who told her is also the person who tipped of Bottoway that Shaylenne was spying for the FBI. So that he'd kill her and produce a threat large enough that Bridget would try something this insane instead of just lying low some place. It couldn't just be some thug it had to be a man psycho with resources. Why does she know which guy in the bar works for Bottoway? She's told.While Bottoway didn't know about Siobhan, Andrew Martin most certainly did. It's a matter of staggering luck that Bridget was able to keep so many people so confused for as long as she did and Siobhan's plans seemed to have been longer in term than I would trust this ruse to hold together. Siobhan is many things but stupid doesn't appear to be amongst them and there was no reason for her to set up a plan this complex when simply making a few large withdrawls and then dying her hair a new color would have saved her from Andrew who doesn't seem the type to hunt you down if you're not a threat and if her goal was money she could make some very large withdrawels and then go with a much simpler plan. If she wants Henry and he wants her once she's safe and comfortable he can leave Gemma anytime he pleases and join her. Nobody as smart as her does this without being manipulated somehow.
The twins will team up next season
As an extention of the above theory. If the show is picked up next season will feature an uneasy quasi-alliance where the sisters are knowingly switching places in an effort both to acheive their goals, for Bridget this will be regaining the family she'd hobbled together out of Siobhan's mess and not dying for Siobhan it's money plain and simple. The two will be trying to peice together what it is that Siobhan and or Bridget either know or own without knowing it. Since it's someone of high enough power to have put something like this together they are probably in the government. Notice they were more than happy to let Agent Machado chase his tail but once he started getting close they kept trying to get him out of the way.
The plan will work well since both girls are good liars. The only thing that will stop them from complete victory is a last second change of heart from Siobhan when she's made an offer.