You can pass an array of layers to the select control
selectControl = new OpenLayers.Control.SelectFeature(
[lineLayer, pointLayer],
{
clickout: true, toggle: false,
multiple: false, hover: false,
toggleKey: "ctrlKey", // ctrl key removes from selection
multipleKey: "shiftKey" // shift key adds to selection
}
);
map.addControl(selectControl);
selectControl.activate();
See the openlayers documentation