Which number to port out first?

Porting out a phone number does not close the account or cancel service on the other phone line. However you will lose control of the account and won't be able to log in if you port out the primary owners phone line first. So I would suggest porting out the secondary line first.

You should also be aware that service is not prorated. So whenever you port the phone numbers out you will still pay for service till the end of the bill cycle. You also don't want to go even one day into a new bill cycle. So please make sure you plan ahead with your new carrier so that you port your numbers out and do not roll over into a new bill cycle. Some providers take as much as 5 days to port your number. For example mint mobile specifically says online port can take 5 to 7 days. So waiting until 2 or 3 days before the end of your bill cycle is a guaranteed fail.

However you will lose control of the account and won't be able to log in if you port out the primary owners phone line first.

That is is not true. The only thing you won’t be able to do is receive a secondary authentication text at that number, if one is required. Otherwise, the primary owner’s access id will work as normal.

Mine did not.  

Thanks all, I’ll port the secondary 1st and then the primary. Also, thanks for the heads up on no prorating, I had forgot about that