combining overlays in mapbox / leaflet layers control

I'm trying to combine 2 or more overlays into one overlay checkbox. I'm using leaflet layers control with mapbox.js 1.6 to toggle my overlays. It doesn't matter to me if I combine them on mapbox.com into one data layer, or if I combine separate data layers in my JS code into one overlay checkbox, but I can't seem to do either. I'm exporting MBTiles from Tilemill to my Mapbox account.

Note that it's not an option to:

• combine them in Tilemill (the single zoom level and square bounding box won't work for me across all layers)
• add the various data layers to a single map project on Mapbox.com (I'd like it to be toggleabe by the user)

You can use L.layerGroup to combine layers

var group = L.LayerGroup([layer1, layer2];

// add default layers to map
map.addLayer(layer1);

// switcher
var baseLayers = {
    "My Group": group,
    // more layers
};

// add layer groups to layer switcher control
var controlLayers = L.control.layers(baseLayers).addTo(map);

You may be interested in this thread Leaflet layer control for basemap group layers