How do you sign in on two different screen names on yahoo at the same time?

My friend and I want to play a prank on my sister by petending to be talking to Criss Angel Mindfreak back and forth. But it wont let me sign in as me and a fake ID I made for the prank. I tried to do a regular messenger and the web messenger at the same time but it just says loading on the web messenger, it wont log on. Where can I go to log on as two different screen names at the same time for yahoo? My sister loves Criss Angel, so thats why we are pranking her. She pranked us a while back and now we ar finally going to get her back!