Really simple, I would have expected bike to behave this way (similar to browsers and other tabbed applications. Another request would be to have a simple way to open an existing bike file in a new tab instead of window. ⌘+T could be utilised here?
For existing discussion see:
Browser-like tab management - Bike Outliner - Hog Bay Software Support
More generally:
And in particular, the standard:
^⇥ Window > Show Next Tab
^⇧⇥ Window > Show Previous Tab
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