Pass Multiple Parameters to jQuery ajax call

data: '{"jewellerId":"' + filter + '","locale":"' + locale + '"}',

simply add as many properties as you need to the data object.

 $.ajax({
                    type: "POST",
                    url: "popup.aspx/GetJewellerAssets",
                    contentType: "application/json; charset=utf-8",
                    data: {jewellerId: filter , foo: "bar", other: "otherValue"},
                    dataType: "json",
                    success: AjaxSucceeded,
                    error: AjaxFailed
                });

Don't use string concatenation to pass parameters, just use a data hash:

$.ajax({
    type: 'POST',
    url: 'popup.aspx/GetJewellerAssets',
    contentType: 'application/json; charset=utf-8',
    data: { jewellerId: filter, locale: 'en-US' },
    dataType: 'json',
    success: AjaxSucceeded,
    error: AjaxFailed
});

---

UPDATE:

As suggested by @Alex in the comments section, an ASP.NET PageMethod expects parameters to be JSON encoded in the request, so JSON.stringify should be applied on the data hash:

$.ajax({
    type: 'POST',
    url: 'popup.aspx/GetJewellerAssets',
    contentType: 'application/json; charset=utf-8',
    data: JSON.stringify({ jewellerId: filter, locale: 'en-US' }),
    dataType: 'json',
    success: AjaxSucceeded,
    error: AjaxFailed
});