Good question actually.  Assuming the quota doesn't expire, it should just stack up, but we can't be sure what TM is doing.

I don't even dare to ask on Live Chat given all the blur sotongs in CS giving conflicting information so far.

The only way to confirm is if someone actually tries to buy 2 add-ons one after another and then feedback to us here.

I only got mine today.  Haven't bought anything yet.  Haven't even used anything yet.

what app is this?

I already collected the sim card and the TM guy told me no active days shown but you can check based on 90 days from the history, there are reload, add on and pay per use.
This will end the 90 days speculation~

Yes, after u click first time login and key in the number it will send the sms for u to key in the verification code.
If u don't receive immediately, switch to airplane mode then back on and it should be able to come in.
2 of my sim cards need to do like that in order to receive the sms.

login with mobile number, not email address
all account are separated individual mobile number login
refer to history, 90 days from the last date of reload, add on or payu