2columns

<div xmlns="http://www.w3.org/1999/xhtml" lang="en"
     xmlns:f="http://typo3.org/ns/TYPO3/Fluid/ViewHelpers"
     xmlns:flux="http://typo3.org/ns/FluidTYPO3/Flux/ViewHelpers"
     xmlns:v="http://typo3.org/ns/FluidTYPO3/Vhs/ViewHelpers">

    <f:layout name="Content"/>

    <f:section name="Configuration">
        <flux:form id="grid66" label="{f:translate(key:'ce.grid.66')}"
                   description="{f:translate(key:'ce.grid.66.description')}" options="{group: 'LayoutElements'}">
            <flux:form.option.icon value="EXT:myext/Resources/Public/img/Backend/CE/TYPO3-BE-icn-grid-50_50.svg"/>
            <flux:field.input label="{f:translate(key:'ce.props.customclass')}" name="customclass"/>
            <flux:grid>
                <flux:grid.row>
                    <flux:grid.column name="col1" label="50%" style="width: 50%"/>
                    <flux:grid.column name="col2" label="50%" style="width: 50%"/>
                </flux:grid.row>
            </flux:grid>
        </flux:form>
    </f:section>

    <f:section name="Preview">

    </f:section>

    <f:section name="Main">

        <div class="row flexbox {customclass}">

            <div class="col-md-6 col-sm-12">
                <flux:content.render area="col1"/>
            </div>
            <div class="col-md-6 col-sm-12">
                <flux:content.render area="col2"/>
            </div>

        </div>

    </f:section>
</div>