Hmm. I've got 3 better bottles I've not been using because I assumed the wine pump would suck the sides in and not work for racking.
I tried it once (I didn't have an extra carboy), and the BB walls collapsed. It did work. The primary was on a counter (higher) and the carboy on the floor. I let gravity do the rest.
Ok to try once, but I would not do it repeatedly. The flexing fatigues the wall and weakens it. Eventually, it will break.
Here is a solution to use plastic carboys for Secondary fermentation...
Purpose: Use AIO to start the siphon into the receiving carboy.
1) Normal tubing from the wine to the receiving Carboy.
2) Right before the neck to the carboy, put a T.
3) On each of the other sides of the T, put a valve.
4) Once side goes to the AIO, the other side goes to the receiving carboy.
Place wine on a counter, and the receiving carboy on the floor.
Close the valve to the receiving carboy, open the valve to the AIO.
Turn on the pump, and let it draw the wine into the tubing.
When the wine reaches the T, close the valve to the AIO, and open the valve to the carboy.
Turn off the pump, and let gravity do the rest.