My guess is that one account is a google apps account, and not all the skins work for Google Apps version of Gmail. =)

You could always set one of the gmail accounts to forward the mail to the other one... and then use the "add account" feature in the settings to let you send mail from the second account.

For instance, I have a bunch of email accounts but I use geek@howtogeek (Gmail Apps) as my primary, all the others forward to that one. Then I have it setup so I can send email from the one account as another (like howtogeek@gmail for instance.)

The only problem is that it shows something like "From: email address on behalf of email address" when sending mail, so people will see both email addresses if they are looking. Otherwise it works out well.