if i delete a back up will it delete all of my stuff or just that one back up

If the other iPad is signed into the same account, has identical data on it you could and is running the same or lower iOS version you could, but I wouldn't risk it. An example that would get you in trouble would be if your other iPad is newer and over time you update it to an iOS version that is not supported on the iPad 2, then you would not be able to restore the backup to your iPad 2. (You can only restore a backup made on a device running the same or lower version of iOS.) It's also unlikely that your other iPad has identical data. They would typically have different app data, and may have different photos, apps and other data.

Deleting an iCloud backup only permanently deletes the backup from iCloud. It doesn't do anything to the data currently on your iPad. However, unless you have a backup of your iPad on your computer, you will be running the risk of losing all your data and settings if you lost your iPad, had to restore it, or wanted to replace it with another device. If you're going to stop using iCloud backup, be sure to start syncing regularly with iTunes to back it up on your computer.

This article explains other options for reducing your iCloud storage: http://support.apple.com/kb/ht4847.

If you are trying to free up space on your iPad, deleting a backup won't help. The backup isn't stored on your iPad, it's stored in your account on Apple's iCloud server. Deleting the backup will free up space in your iCloud account, but not on your iPad.

If you want to free up space on your iPad, you'll have to sync some data off it (such as some of your music) using iTunes, or import your photos and videos to your computer and delete them off your iPad as explained here: http://support.apple.com/kb/HT4083.