var wrapper1 = document.getElementById("signature-pad-1"),
    canvas1 = wrapper1.querySelector("canvas"),
    signaturePad1;

var wrapper2 = document.getElementById("signature-pad-2"),
    canvas2 = wrapper2.querySelector("canvas"),
    signaturePad2;


function resizeCanvas(canvas) {
    var ratio =  window.devicePixelRatio || 1;
    canvas.width = canvas.offsetWidth * ratio;
    canvas.height = canvas.offsetHeight * ratio;
    canvas.getContext("2d").scale(ratio, ratio);
}

resizeCanvas(canvas1);
signaturePad1 = new SignaturePad(canvas1);

resizeCanvas(canvas2);
signaturePad2 = new SignaturePad(canvas2);

var wrapper1 = document.getElementById("signature-pad-1"),
    canvas1 = wrapper1.querySelector("canvas"),
    signaturePad1;

var wrapper2 = document.getElementById("signature-pad-2"),
    canvas2 = wrapper2.querySelector("canvas"),
    signaturePad2;


function resizeCanvas(canvas) {
    var ratio =  window.devicePixelRatio || 1;
    canvas.width = canvas.offsetWidth * ratio;
    canvas.height = canvas.offsetHeight * ratio;
    canvas.getContext("2d").scale(ratio, ratio);
}

resizeCanvas(canvas1);
signaturePad1 = new SignaturePad(canvas1);

resizeCanvas(canvas2);
signaturePad2 = new SignaturePad(canvas2);

<button id="clear1">Clear</button>
<button id="clear2">Clear</button>
<button id="save">Save</button>

function clear1() {
    signaturePad1.clear();
}
function clear2() {
    signaturePad2.clear();
}
function save() {
    if (signaturePad1.isEmpty() || signaturePad2.isEmpty())
        alert("Error: Please sign both pads!");
    else
        alert("Success!");
}

$("#clear1").click(clear1);
$("#clear2").click(clear2);
$("#save").click(save);