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Firefox Nightly on Android currently supports Split Screen and "window" view.
If you don't mind daily updates (pun half-intended) and possible instability, then you could download it.
Otherwise, there's the unfortunate risk that this feature will be kept locked in Nightly for an indeterminate amount of time, along with other features such as "Pull Down to Refresh" or about:config...
Does this include the ability to open a new window? I know in Chrome, Chromium, and Bromite for Android you can open a new window. This is helpful for streaming sites that allow you to open the chat in a new window, roll20, and a whole host of sites that work best when split into multiple windows rather than tabs.