This doesnt work?..
Hi there, @Madelaine_Rascan 
That code is for our Slider app, this is why it didn’t work in your Testimonials Slider widget.
To show the part of the next slide on mobile, click on the Customize button on the Layout tab. then choose Custom option for the Columns setting and set 1.2 for the Mobile view:
Please check it out and let me know if it helped 