I am not too sure if this works for all plane types given that there are different service provider. I just tried it on the A350 plane from HND to SIN today. I used the complimentary wifi for Biz Class/PPS Club members and I login using my email address and password (instead of logging in as a guest). After i login to the first device, I went to singaporeair.com website and was able to logout.

I proceed to 2nd device and was able to login using my email address and password (that I login in the first device) and the remaining data quota was there for me to use.

My travelling partner forgot to logout from the first device and still able to login to 2nd device to surf the net. I guess they have auto logout his first device upon logging to 2nd device.

Hope this observations help.