Is it possible to merge two email accounts together

if the account you want to drop doesn't have a huge amount of mail/attachments that you want to keep, (and both are set up as IMAP) I would just drag and drop all mail from that to the one you want to keep (in fact I did this just after I migrated to KH) having first setup the forwarder. Then you can safely delete the unwanted account.
There is one more solution to merge two and more emails account into single mail. I hope this will help for you.

Just a note for anyone that reads this looking for solutions.

The above would only work if they were trying to merge mail locally on the computer.

Web-based mail services don't utilize the .pst function as mail client software does.

If I'm not mistaken, the above referenced link would only work for Outlook too, as I believe they are the only ones that utilize '.pst' as their default format.