One solution I have seen in the past, was to use javascript to open a new window:
# Open the first browser window as normal
b = Watir::Browser.new
b.windows.count
#=> 1
# Execute javascript to open a second window
b.execute_script('window.open();')
b.windows.count
#=> 2
Note that there is a single Watir browser object, but it knows of 2 windows.