I'm sorry about the loss of your husband.
I wonder if, after he passed away, your kids began to pull together (as they should) and lean on each other for comfort, and now it's become a habit. Perhaps they developed a pattern that now should progress to different behaviors. If they're still grieving and haven't been able to come to terms with the death of their daddy, maybe they need some counseling. Of course they'll always miss him, but four years have passed and it sounds as though they are still freshly grieving him instead of accepting the loss.
Being close is nice. However, sharing the bed at the age of 17 is extreme.
I think you definitely should wake them up if they fall asleep on the couch and send them to their separate rooms. Sleeping on a couch is a poor habit to develop. Sleep hygiene, as the doctors call it, requires using the bedroom for sleep, sleeping on a proper bed and mattress, and other routines (no sleeping with the tv on, keeping the bedroom comfortably cool and dark, etc). Wake them up and send them to their beds.
Talk to them about not sharing the bed anymore. That's a childish thing, and maybe they needed it four years ago, but now they need to establish independence. If one is upset, tell them to go to the kitchen and make mugs of warm milk or cocoa and talk things out and then return to bed. A sleep doctor will tell you that using the bed to watch tv or to have family conferences will lead to poor sleep patterns. Of course, that doesn't mean there can't be occasional times when the whole family gathers on the bed, for a fun pillow fight, or to serve mommy breakfast in bed, or some special occasion, but those occasions should be rare, not routine.
Often people who are not twins, or who don't have siblings, cannot understand that bond. And your twins have a deeper bond than simply being twins: they're twins who experienced a loss. Acknowledge it, but help them to move on in some respects (the sleeping on the couch, the sleeping together). Enjoy the fact that they are good siblings to their younger brother, that they love each other, but help them develop a little more adult behavior in the sleeping department.