Wamsy, which smartfly board are you using? I have the power expander pro in my 89.
Gary, you can do the same thing with a Fromco Wolverine switch... great product to have
I'm thinking about getting one for their batt share units to use in my 30cc project...
For 25% i just use one rx battery and have never had a problem. I use two battery's in 30% and up.
Remember, there's no specific reason to use redundant batteries in one size vs another. It's all about how much chance you want to take on a single battery.
How often does a battery fail?
The REAL concern is getting enough power to your servos. As long as you have appropriate connectors on your battery and some sort of power bus...then one battery is perfectly fine on a 35% plane...as long as you're ok with taking that chance. It just so happens that most power board/bus devices have a batt share feature built in.
All that being said I run dual batteries in 50cc and up. In my 30cc sized planes, I make sure I have 2+ leads coming off of my switch so I can get more then 3A of power to the Rx.