Oh my! I'm sorry, but that's just crazy! lol...how in the world is anyone getting any good sleeping? Kids will get away with what you ALLOW them to get away with. I don't mean to sound harsh...but...you need to nip this in the butt and finally stick to your guns. You do your nighttime routine...read a story, etc., love on them each...then, it's time for bed. Are there other factors, such as scared of monsters or anything like that? if so, there are some cute ideas for dealing with that...but if you feel this is truly just a battle of the wills, I would say, enough is enough. Right now they are in control and that is not healthy for anyone involved. I know that's hard to read, but they are surely calling the shots for you to have to go to such lengths.
I personally would make sure there is a gate placed at their door (two stacked on top if need be, if they are climbers, lol)...this handles any potential unsafe night roaming...as well as they actually might feel a bit safer, they can see what is going on outside of their room, yet there is a 'barrier'. They are certainly old enough at this age to comprehend...it's bedtime, we are going to sleep and we are doing so in our own beds. I also think from my own experience they are past the prime 'Ferberizing' time...and that the more attention you give their tantrums, the more prolonged it will be. I would occasionally say "it's time to go night-night (or insert your term here), I love you, now go get in your bed" I would explain that I will only tuck you in once...if you do not stay in your bed you will have to retuck yourself in. End of story. They may even fall asleep on the floor for a few nights...but trust me, they'll get the hint and realize whose won this battle, lol.
I usually go things hard core and just get it done...but I've heard of some people using graphs/charts...if say they sleep in their bed for 'x' amount of days they will receive some form of 'reward'.
*****EDIT**** sorry, I just realized you gave an update and that you have solved the problem, great job guys!! enjoy your peaceful sleep in your bed!! :o)