There's a couple good tricks for this. It's amazing how much you can lose where you are when you are going fast like that. One way to figure it out is timing. If you know well enough how long it takes to do a single front flip, when you are going for your double you just count it, one-two-open. Another is not really trying to see everything and know exactly where you are but focus on the colors around you. Gymnastic facilities use colors for this and you've probably never even noticed before. If built right (and it'd be really dangerous if it wasn't) the ceiling will be some neutral grey or white color, and the floor will be some ridiculous blue, red, or bright yellow. When you are doing a double flip you just have to know what color you are looking at. If you see that bright crazy color flying by, you know it's the floor and you can count it passing you by before you know to put your feet on it. The last way almost seems like cheating sometimes but if you have a friend who understands when you need to open the tuck to land it properly, just have them yell at you mid flip. Tell them to yell something quick and loud (usually we just use "HUP") right before you need to open. When you hear them, you open up and land. After a while you'll learn how this timing feels and you won't need the yell anymore.
