Each field row is a div one after another. It's possible to use entirely CSS to split the form like that. You could use flex, grid, or floats even. Example CSS as follows using flex.
Code:
.cbRegistration .cbFieldsContentsTab {
display: -ms-flexbox;
display: flex;
-ms-flex-wrap: wrap;
flex-wrap: wrap;
}
.cbRegistration .cbFieldsContentsTab > .row {
-ms-flex: 0 0 50%;
flex: 0 0 50%;
max-width: 50%;
}
You can add your custom CSS to CB using CSS overrides functionality below.
www.joomlapolis.com/blog/kyle/18711-template-css-overrides-made-easy