Seu código deve ficar assim.
app.get('/api/task_details',function(req,res,err){
var sql = "select * from users inner join user_level_task on users.user_id=user_level_task.user_id inner join tasks on tasks.task_id = user_level_task.task_id where task_name = 'game'";
var sql2="select * from tasks where task_name='game'";
var res1 = {}
db.select(sql,function(err,data1){
db.select(sql2,function(err,data2){
res1.data1= data1;
res1.data2=data2;
res.end(JSON.stringify(res1));
})
})
})
