shift-keys on both right and left side each

REMINGTON TYPEWRITER.
On the Remington typewriter the capital. letters and corresponding small letters are both on the same key. The capital is printed by moving the carriage over just far enough to change the printing point. The carriage is thus moved by means of a key, which is called the shift key. This key must be depressed and held down while the desired letter is being struck with another finger. On the latest machines of this style, there are usually two shift keys, one on each side of the keyboard. In this case one hand is used for the shift key and the other for the letter which is to be written. In this way, the fingering for the capitals is exactly the same as for the small letters.
When there is but one shift key, use the fourth finger of the left hand on the shift key. This will make it necessary to change the fingering somewhat for the capitals on the left hand side of the keyboard. Hold the shift key down with the fourth finger, and strike the letter with one of the other three fingers, as most convenient.

Manual of Dougherty's brief shorthand and touch ... . - Full View | HathiTrust Digital Library:

For my own use, I prefer a single case for its compactness, and the machine should have a capital shift on the right hand side as well as on the left.

TOUCH TYPEWRITING. (Continued.) BY FRANK RUTHERFORD.
McKee's shorthand magazine., Vol. 2, No. 3, January 1901, P.50.
McKee's shorthand magazine. v. 2 (Nov. 1900-Oct. 1901). - Full View | HathiTrust Digital Library:

Daugherty, Pitsburg underwood.