You must first understand how aggregation works. If you know Linux, it works very similarly to pipes. The results of one pipeline calculation can be used in the next pipeline. But if you use it in the same pipeline, you can only recalculate it once

db.getCollection('XXX').aggregate({
    $project:{
        _a: {$multiply:['$a', 2]},
        _b: {$add:[{$multiply:['$a', 2]}, '$b']}
    }
});

Or do calculations in the next pipeline

db.getCollection('XXX').aggregate({
    $project: {
        _a: {$multiply:['$a', 2]},
        b: '$b'
    },
    $project: {
        _a: '$_a',
        _b: {$add: ['$_a', '$b']}
});

Solved, it works like this

db.getCollection('XXX').aggregate({
    $project:{
        _a: {$multiply:['$a', 2]},
        _b: {$add:[{$multiply:['$a', 2]}, '$b']}
    }
})