It is best to connect all accounts with the same institution at the same time. If you have already connected one account but want to add another with the same bank, it would be best to disconnect bank feeds on the connect account and then add both together. It is very important to clear all unaccepted bank feeds BEFORE disconnecting the bank account and here is why: your bank may well send duplicate transactions for that previously connected account. It is easy to mark all the duplicated transactions to be excluded for that account when you reconnect and if disconnect and reconnect happen on the same day that will prevent duplicates from getting into your books. Make the new account in your chart of accounts before reconnecting the whole family of accounts in one session.
Once you have all accounts with that bank connected, remember to treat money flows between them as transfers and matches so you don't duplicate payments and transfers - same for credit card payments pulled from bank accounts.