Couple of different options. There is a FF addon called gmail addon that makes it easy to switch. You could do the "forwarding" route which resends the email to your master account. You can add accounts to your gmail account, which will fetch via IMAP/POP3 all the emails from one to the other. However, the latter 2 options do not keep the accounts separate (although you CAN send email from either account).
Why do you want to keep them separate? If it's because they're shared and you want to separate access, I suppose you could create a third gmail account (call it username-master or something) which would fetch from BOTH your current existing accounts, without muddying either of them as they are.