Scratch

I'm trying to make a side scroller game but I can't figure out how to do it and need a SIMPLE explanation!! Everytime I ask I get a complicated answer and no one can give me a script that works    I want it to be like in Super Mario Bros is possible. What will I need to do? My game starts off in a library so is there a way to repeat the bookshelves to move like I'm moving across the stage?

Second problem is my 2 players. One is controlled by the arrow keys and the other is controlled by A,W,D,Space Bar. The problem is for some reason they cancel each others actions out. When I try to move them both at the same time, one of the characters will get cancelled out by the other as it carries on walking    How do I stop this??? It's so frustrating and really setting me back on my deadline!! (It's my school project)

Lastly I'm having a really big problem with my jumping sprite. I've tried so many different Scratch Block combinations but none of them seem to work how I want them to! Basically when I click the up arrow (to jump) the costume for jumping will either not show or flash for 1 second. And that's not the only problem. Right after jumping I can't carry on walking D: I want to be able to jump over obstacles and then carry on walking after as normal, not have to press the walk key down again!

If someone could give me the scratch blocks needed to get this working, it would be much appreciated.

Incase Needed, I have (for each character):
8 walking costumes
1 jumping costume
2 shooting costumes (It's a shooting game)