A few things.
- fix your formatting. It's a mess. Basic indentation goes a long way to understanding what's going on. Curly braces on their own lines (opening braces at the end of the line where the block starts is fine). No more than one curly brace on a line. Then press ctrl-T in the IDE and it'll look a lot nicer.
- use proper names. Name your pins, name your variables properly. That makes life so much easier (and your code much more maintainable than having to hunt down all instances of pin 10).
- look up variable types, and how they work. Double is a floating point type, and the wrong type to store a large integer like you get from millis(). The way a float is stored may also cause problems when you try to work with those times. You need an unsigned long for that. By the way, double and float are aliases on Arduinos.
When you follow those basic practices you have a much better chance of understanding what you're doing.
Mikeh23:
Is this possible with one Arduino or will two have to be used
A single Arduino will be basically idle >99% of the time with the work load of two stop watches and an LCD screen.