How to do an ASCIIpOrtal like portal effect

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:bust_in_silhouette: Asked By Hsingai Tigris Altai

How can I draw a different scene in the shadow if a portal? Bonus points if you can figure out a way to draw each side of the borders of the shadows a different color like in the mock up.

My idea was to use viewports to draw each scene then mask them off somehow and then draw the borders ontop on it.