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
});