JavaFX - Multiple nodes in one Grid Cell with specified width (in percentage possibly)

Question

So I'm trying to add multiple Nodes in one Row of a GridPane with sizes in percentages. This is the UI, with the Grid Lines visible:

When the user clicks the Checkbox for "Check Editbuffer Checksum", I want to display a ComboBox next to Checkbox in the same Row and Colum. My problem is, that the Combobox fits the entire Cell, so the Checkbox is not visible anymore. Basically, it should look like this:

And if the user deselects the Checkbox the ComboBox should disappear again. This is the code to create the GridPane:

private void fill() {
    getChildren().clear();
    getRowConstraints().clear();

    setMinWidth(350);
    setPadding(new Insets(17));
    setVgap(5);
    setHgap(5);

    lTitle.setAlignment(Pos.CENTER);
    fxAssign.setMaxWidth(Double.MAX_VALUE);

    RowConstraints heigthPadding = new RowConstraints();
    heigthPadding.setPercentHeight(40);
    add(lTitle, 0, 0, 2, 1);
    getRowConstraints().add(heigthPadding);

    addRow(1, lName, fxName);
    getRowConstraints().add(new RowConstraints(ROW_HEIGHT));
    addRow(2, lRequest, fxRequest);
    getRowConstraints().add(new RowConstraints(ROW_HEIGHT));
    addRow(3, lHeader, fxHeader);
    getRowConstraints().add(new RowConstraints(ROW_HEIGHT));
    addRow(4, lLength, fxLength);
    getRowConstraints().add(new RowConstraints(ROW_HEIGHT));
    addRow(5, lForm, fxForm);
    getRowConstraints().add(new RowConstraints(ROW_HEIGHT));
    addRow(6, lCheck, fxCheck);
    getRowConstraints().add(new RowConstraints(ROW_HEIGHT));

    add(fxAssign, 0, 7, 2, 1);
    getRowConstraints().add(new RowConstraints(ROW_HEIGHT));

    ColumnConstraints columnConstraints1 = new ColumnConstraints();
    columnConstraints1.setPercentWidth(50);
    ColumnConstraints columnConstraints2 = new ColumnConstraints();
    columnConstraints2.setPercentWidth(50);
    getColumnConstraints().add(columnConstraints1);
    getColumnConstraints().add(columnConstraints2);

    setGridLinesVisible(true);
}