Multiple-Column Lookup value is represented as an array of SP.FieldLookupValue objects.

How to read multiple Lookup field value

var context = new SP.ClientContext.get_current();

var web = context.get_web();

var list = web.get_lists().getByTitle(listTitle);

var listItem = list.getItemById(1);   

context.load(listItem);

context.executeQueryAsync(

   function() {

       var lookupVals = listItem.get_item(fieldName); //get multi lookup value (SP.FieldLookupValue[])

       for(var i = 0;i < lookupVals.length;i++) {

           console.log(lookupVals[i].get_lookupId()); //print Id

           console.log(lookupVals[i].get_lookupValue()); //print Value

       }

   },

   function(sender,args){

       console.log(args.get_message());

   }

);

How to update multiple Lookup field value

For updating multiple Lookup value you need to specify value of type SP.FieldLookupValue[]. Note, SP.FieldLookupValue could be initialized by specifying LookupId only.

var context = new SP.ClientContext.get_current();

var web = context.get_web();

var list = web.get_lists().getByTitle(listTitle);

var listItem = list.getItemById(1);   



var lookupVals = [];

//set 1st Lookup value

var lookupVal1 = new SP.FieldLookupValue();

lookupVal1.set_lookupId(1);

lookupVals.push(lookupVal1);

//set 2nd Lookup value

var lookupVal2 = new SP.FieldLookupValue();

lookupVal2.set_lookupId(2);

lookupVals.push(lookupVal2);



listItem.set_item(fieldName,lookupVals);

listItem.update();



context.executeQueryAsync(

   function() {

        console.log('Multi lookup field has been updated');

   },

   function(sender,args){

       console.log(args.get_message());

   }

);