If your lo is satisfied and content between feeds on just one side, then that's all she needs.
Basically the current advice is to let lo nurse on the first breast for as long as she'll go naturally. When she comes off herself, offer the first side again. When she's flat out refusing to go back on the first side, offer the second. You'll then learn if it's worth offering the second side...
With DD1 I had to double side feed due to supply issues. With DD2, the only time she's taken from the second side is during growth spurts.
What makes you think you've had a decrease in supply? Check out:
http://babywhispererforums.com/index.php?topic=65905.0If you are definitely experiencing a supply issue (and that's always assuming that the decrease isn't just your supply adjusting as it's about the time that it'd become properly established), then double side feeding or even switch nursing (going back to the first breast when baby has finished with the second), will help to boost your supply.
HTH
Happy New Year!
Jo x