That was not what I was trying to point at.
If you open a new tab with $newtab = $psise.powershelltabs.Add(), the new tab will be a new PS session.
As a consequence the $global:defaultViServers there is empty.
You can connect to another vSphere server in the new tab.
Just tested that, and it works without an issue, one tab with a connection to vCenter1 and a second tab with a connection to vCenter2.
Blog: lucd.info Twitter: @LucD22 Co-author PowerCLI Reference