Hey girl, hows it going? I would just put her in a seperate container, just WHILE she eats, then put her back in the tank.
Now, I say that thinking you are there watching, but if you can't be there, then, if shes really that thin, yeah, put her in another tank, until she beefs up if Bam Bam is eating all the food.
As far as the uvb, depends on how long you keep her seperated. If it were just briefly, my opinion is no you would not have to buy another light.
But if it is going to be an extended period of time, yes.
** Oh and somewhere down the line you may want to have a spare anyway, just incase you would ever have to seperate them, for a long period. Always good to have a back up set up (like a hospital or sick tank, for any emergencies) ** 
Eve