Are there any advantages/disadvantages between the two in a suspended ceiling application? I have standard fixtures laying around I could use, but I'm guessing I would need some sort of special extended box to use with the ceiling. Appreciate any thoughts.

Last summer installed a 200 amp 40 breaker Square D Homeline panel box. After I wiring my renovated house, I was coming very close to using all 40 breakers. Tried to use a few tandem breakers, but discovered the Homeline box won't accept tandem breakers. Therefore, decided to add a subpanel. I installed another 200 amp 40 breaker Square D Homeline panel. I know I don't need another 40 breakers, but I figured that if I decide to go up to 400 amps in the future (a pool and spa are in the not too distant future), I would be able to just sever the subpanel cable and be ready to go. I installed a 240 volt 50 amp double pole breaker from the main to the subpanel and ran the line to another 240 volt 50 amp double pole breaker in the in the sub. I have three questions. First: My understanding is that a subpanel needs to have a separate ground and neutral, so I have all of my grounds running to one busbar and all of the neutrals to the other. The problem is that I can't figure out how to separate the bars at the top of the box. It appears to me that the bars are connected at the top where the line would come in if it was not a subpanel. There is a phillips head screw that connects the busbar to the top cross bar, but it is either torqued down so tight or fastened some way since I can't budge either one of them. What am I missing here? Second: I live in Virigina. I am assuming that I need to ground the box buy installing the grounding screw in the subpanel as well. Third: I thought it would be a good idea to run the line from the main to the sub using two single pole 50 amp breakers to draw from both sides of the main bus. Couldn't find any single pole 50 amp breakers at the local warehouse. Should I do as I planned or is using a double pole breaker OK?