Has there ever been a sideways H-tail on an airplane?
Most of the initial aircraft made by Wright brothers had the twin horizontal stabilizer setting. However, the elevators in these cases were set in the front, rather than back, as is the norm now.



By Wright brothers - OhioLINK Digital Media Center, Public Domain, Link

Quite a few of the early aircraft had a 'box' like configuration, notably the Santos-Dumont 14-bis, which made the first public powered flight. Again, though the pitch control was through the box like structure, it was more of a canard than a horizontal stabilizer.

 

Santos-Dumont 14-bis; image from Wikipedia

One of the earliest aircraft with a 'proper' twin horizontal (as well as vertical) stabilizer was the aptly named Bristol Boxkite



Bristol Boxkite; By Hpeterswald - Own work, CC BY-SA 3.0, Link

Another good example is its predecessor, the Farman III, which had two horizontal stabilizers and a single vertical tail right in the middle, forming a perfect sideways 'H'.



Farman III; By C.C. Pierce &amp; Co. - http://digitallibrary.usc.edu/cdm/ref/collection/p15799coll65/id/12574, Public Domain, Link

You can see that most of these are early aircraft, which had structural limitations. As the aircraft design matured, the 'conventional' layout became the norm as a single stabilizer assembly can be used to achieve the necessary pitch control.