As for the boards:
You can have both daughters learn to ride on the same board. Because daughter 1 might grow past 140 Lbs, I'd suggest getting a weight range on the board that would accommodate that growth. That being said...go for one that accommodates more than 140 Lbs so you don't have to turn around and buy another board right away. Below are suggestions for female specific boards for beginner riders.
2006 Liquid Force Diva 134 is a great beginner board. It accommodates rider weights from 90 Lbs to 160 Lbs. It's a stable shape that provides a smooth, flowing, and predictable feel. Pretty good tracking on jumps and landings and a gentle release off of the wake.
2006 Hyperlite Eden 130 is also a great beginner board. It accommodates rider weights up to 165 Lbs. It is also stable, smooth, and good for learning on.
2006 O'Brien Vixen 137 is a bit larger of a board but a good beginner board that would accommodate the weight range of both of your daughters. It accommodates rider weights of 100 to 170 Lbs, and is also stable, smooth, and good for learning on. I tend to ride shorter boards myself (5'4" and 120 Lbs) so this one appears to be quite large as my boyfriend rides 134 to 138 and is 5'11" and ranges between 165-175 Lbs.
Hope this helps. Any other questions, please ask. Congratulations on your daughters getting into wakeboarding.
Check out
www.girlslearntoride.com some time.