For smaller size aircraft, the load on the battery pack is not as great as it is for larger, even 40 size planes. Where the redundancy comes in is with larger aircraft with 5 or 6 servos and a good load on the control surfaces while doing maneuvers.

I have two 90 size pattern planes and use the parallel battery approach. Both have OS 1.20AX engines... weight change from one to two battery packs in negligable. I have spare channels on the receiver so one battery goes to the battery port and the other goes to a spare receiver slot. Or, you can always use a heavy-duty Y cable and paralled them that way. Each battery has it's separate switch.

Ok, that's the basics. Weight wise, well, as you said, smaller, say 25 size planes are probably not good candidates for parallel battery packs because and added several ounces might make a difference on how that smaller plane flys and handles. As you move up in size and power available, that amount of weight becomes less and less of a concern so it becomes a point of experimentation. If it works, go for it. If you don't like the way it handles with the added weight, well, remove it and go to a larger battary pack.. say from 1200 to 1800 mah pack.

CGr.