Up until recently I have been able to have 2 yahoo email accounts open and switch to both now I can't any idea?

Use Different Browsers. if you use the same browser, the cookies set by the mail account may conflict.





A recent update in Internet Explorer had some modifications on cookies done so that may have been the start of your problems.





I get the same issue on Gmail, so I use Safari for one account (it does not share the Windows Folder of cookies) and IE for the other account.





Hope this helps