Add two methods
@UiChild
public void addLeft(Widget w) {...}
@UiChild
public void addRight(Widget w) {..}
Then make sure you widget implements HasWidgets or similar.
In your uibinder you can now use
<custom:mywidget ui:field="widget">
<custom:left><!-your widget><custom:/left>
<custom:right><!-your widget><custom:/right>
<!- your widget>