There are many ways, correct ways to display that kind of information, but making it a 100px fixed height with a vertical scroll bar doesn’t seeem right.
But it’s your site, so you make the call.
You can have what you want in 1 widget by wrapping the area with a div that has fixed height and overflow-y set to auto, since it’s a text widget so you can just use inline CSS for that.
<div style="height:100px;overyflow-y:auto;">
(content limited to 100px height and a vertical scroll bar)
</div>
and yes, you can have it in 2 widgets and adjust CSS to make it look like 1 but you got to be able to put this in the lowest part of your child theme’s style.css
#black-studio-tinymce-3 {
padding-bottom: 0;
padding-top: 0;
margin-bottom: 0;
border-bottom: none;
border-radius: 4px 4px 0 0;
}
#black-studio-tinymce-7 {
border-top: none;
border-radius: 0 0 4px 4px;
}