Theophylact of Ohrid

In like manner, of the poor as well, who are signified by those grinding, one will be saved and another will not. For not all the poor are righteous: some of them are thieves and cut purses. By grinding is indicated the laboriousness of the life of the poor.

Commentary on Luke ↗

Ambrose of Milan

Or out of the same bed of human infirmity, one is left, that is, rejected, another is taken up, that is, is caught to meet Christ in the air. By the two grinding together, he seems to imply the poor and the oppressed. To which belongs what follows. Two men shall be in the field, &c. For in these there is no slight difference. For some nobly bear up against the burden of poverty, leading a lowly but honest life, and these shall be taken up; but the others are very active in wickedness, and they shall be left. …

Catena Aurea by Aquinas ↗

Bede

There will be two grinding at the mill. He calls those grinding at the mill who are placed among the people and are governed by teachers, doing the things of this world, whom he also signifies by the name of women, because, as I said, it is expedient for them to be governed by the counsels of the skilled. And he said grinding, on account of the circle and cycle of temporal affairs. Yet he said in one grinding, inasmuch as they provide for the uses of the Church out of their own resources and businesses. …

On the Gospel of Luke ↗

Bonaventure

Secondly, as regards the state of actives, he adds: Two shall be grinding together, through engagement in the active life, which is rightly understood by the millstone, because those who are occupied with the acts of the active life go around and run about in the manner of a millstone and do not stop at one work, as was said to Martha above in chapter 10: "Martha, Martha, you are anxious and troubled about many things." Or because such persons ought to be raised up by the hope of eternal rewards and cast down by the terror of …

Commentary on Luke, Chapter 17 ↗