foo.js

var map = L.mapbox.map('map', 'nico-suonper.map-6gwti04q').setView([-34.92, -57.95], 13);
var features = [];

$.ajax({
    type: 'get',
    dataType: 'json',
    url: 'ajax.php',
    data: {valor: valor1},
    success: function(json) {
        for (j = 0; j < json.length; j++) {
            y = json[j].posy;
            x = json[j].posx;
            color = '#F70707';
            features.push({
                type: "Feature",
                geometry: {
                    type: "Point",
                    coordinates: [y, x]
                },
                properties: {
                    title: 'DATOS',
                    description:
                        datos,
                    "marker-color": "#5dbfe1",
                    "marker-size": "small",
                    "marker-symbol": "star"
                }
            });
        }
        secondFile();
    }
});

function secondFile() {
    $.ajax({
        type: 'get',
        dataType: 'json',
        url: 'ajax2.php',
        data: {valor: valor2},
        success: function(json){
            for (j = 0; j < json.length; j++) {
                y = json[j].posy;
                x = json[j].posx;
                color = '#F70707';

                features.push({
                    type: "Feature",
                    geometry: {
                        type: "Point",
                        coordinates: [y, x]
                    },
                    properties: {
                        title: 'DATOS',
                        description:
                            data,
                        "marker-color": "#5dbfe1",
                        "marker-size": "small",
                        "marker-symbol": "star"
                    }
                });
            }
            map.markerLayer.setGeoJSON({
               type: "FeatureCollection",
               features: features
            });
        }
    });
}