Hello Anti Nuga,

     here is the way to change look of many2many_checkboxes widget's view from single column to two-column.

You have to change single line in xml file of web module.

    path: web >> static >> src >> xml >> base.xml

<t t-name="FieldMany2ManyCheckBoxes">
    <t t-foreach="widget.get('records')" t-as="record">
        <div>
......

to
 
<t t-name="FieldMany2ManyCheckBoxes">
    <t t-foreach="widget.get('records')" t-as="record">
        <div class="col-xs-6">

Hope this will help you, Thanks!