Hemingway use the number two for two purposes (no play on words intended). The first relates to the American man and the girl, whom he calls Jig, being a couple. This reinforces the man's perspective that he wants their relationship to stay just as it is: traveling, staying for a night or a few nights at different places to see different things, and tasting new drinks. The instances where two represents their relationship in this way are these:
‘Yes. Two big ones.’
The woman brought two glasses of beer and two felt pads.
‘We want two Anis del Toro.’
The girl looked at the bead curtain, put her hand out and took hold of two of the strings of beads.
The woman came out through the curtains with two glasses of beer and put them down on the damp felt pads.
The second relates to the division between them--the separateness that Jig's pregnancy is pushing them into. This also represents a significant irony and paradox in that it is ironic that, while a couple, they are divided and separated, and it is a paradox that two can be both separateness and unity. Of course, the answer to the riddle of the paradox is that two going in the same path is unity while two in divergent directions is separateness. These quotes represent two as separateness:
the station was between two lines of rails in the sun.
It stopped at this junction for two minutes
He picked up the two heavy bags and carried them around the station to the other tracks.
In an ironic statement, Hemingway uses the last reference to two to enlighten us on the man's perspective and give the reason for their growing separateness. The man carries "the two heavy bags" round the corner of the station, then looks up the tracks for the train: he "could not see the train." This is a metaphor for the man's experience: he can't see the metaphorical train that is about to hit him. The train symbolizes the disagreement about the abortion that is threatening a collision and will wreck their relationship when it finally arrives.