Do you have a tumble track? I've seen coaches spotting doubles on that. Every time I've seen someone learning double backs it's been with a spotting coach, and a large part of what they do is push the gymnast up a bit, so they have extra height/time to complete the second tuck. So, a pit shouldn't matter. A trained coach and a proper mat should be sufficient.
I'm sorry to say it, but based on the videos you've posted in another thread, you're nowhere near ready to be working double-backs. Before you even think about working doubles, you must have a very tight and fast backhandspring, and a tuck that goes well above head height.