Question about Outlook Express

I own two separate domains, one that I use for personal stuff and another that I use for work, let's call them home.com and work.com. Until a couple of days ago I had these both setup for mail using Google Apps and used the multiple account sign-in feature to access both at once.

Seeing as I recently migrated to a Windows Phone I figured I'd give outlook.com a try and setup work.com as a custom domain under my existing Live ID, which is attached to my home.com address. Fine, that worked, it then asked me to setup a new work.com user to create an email address. I did that and started using the account. All well and good.

The next day I needed to sign in to my home.com account to redeem a code for Xbox Live, and this is where I found that it was impossible to login to the two accounts at once, and I would need to sign out of my work.com account first. Delving deeper I discovered that this feature was recently removed in favour of email aliases. No problem, I thought, and once I'd signed into my home.com account I tried to add a work.com alias.

"You can't add an alias with the domain work.com. Try an email address that has a different domain."

Further digging leads me to believe that this is because work.com has been setup as a custom domain, and even though this account is the very same account that created the custom domain, it can no longer be used for aliases.

So my question is: How can I undo this situation so that I can create a work.com alias for my home.com account, yet still send and receive emails to and from the work.com address without it adding "on behalf of" or anything similar to my emails?

If it can't be done then I guess I'll have no choice but to return to Google...

Thanks for any help.