On a more serious note, I think you need to consider the possible outcomes you want to allow as a DM and rule accordingly.
I allow an enemy and a PC to involuntarily share a space if one of them is prone. While they share the space, all attacks and ability checks are at disadvantage. (As soon as the prone person stands up, they move to an adjacent space.) If the player wants to try and drop prone and crawl through the enemy square I smack them. (The player, not the PC.)
Given that, the outcomes I can see from the attempted shove are:
I would allow the shove attempt, and if it succeeds against the character, then a second shove attempt is made with disadvantage against the ally behind him.
Possible outcomes:
a) Shove fails against the first character, no change in positions.
b) Shove succeeds against the character, fails against the ally. Character is prone in their square, and the enemy can move into their space.
c) Shove succeeds against the character, and succeeds against the ally. Character is shoved back 5 feet and is prone in the allies square.
In a narrow crowded hallway, it is possible to be knocked prone and be unable to stand up as people trample you.
I allow an enemy and a PC to involuntarily share a space if one of them is prone. While they share the space, all attacks and ability checks are at disadvantage. (As soon as the prone person stands up, they move to an adjacent space.) If the player wants to try and drop prone and crawl through the enemy square I smack them. (The player, not the PC.)
Given that, the outcomes I can see from the attempted shove are:
I would allow the shove attempt, and if it succeeds against the character, then a second shove attempt is made with disadvantage against the ally behind him.
Possible outcomes:
a) Shove fails against the first character, no change in positions.
b) Shove succeeds against the character, fails against the ally. Character is prone in their square, and the enemy can move into their space.
c) Shove succeeds against the character, and succeeds against the ally. Character is shoved back 5 feet and is prone in the allies square.
In a narrow crowded hallway, it is possible to be knocked prone and be unable to stand up as people trample you.