[{"field2":1,field1:"value1",field3:"value2"},{"field4":2,field1:"value1",field3:"value2"}]

db.collection.aggregate([{$facet:{query1:[{$match:{field1:"value1"}},{$group:{_id:null,total:{$sum:"$field2"}}}],query2:[{$match:{field3:"value2"}},{$group:{_id:null,total:{$sum:"$field4"}}}]}},{$project:{total:{$add:["$query1.total","$query2.total"]}}}])