There are ways around this problem that I have either used or seen used.
In order of preference:
1, wire each prox back individually. This is not the question you asked, but it is my preferred way of doing things.
2, Wire each prox individually to a relay, then connect the relays in either series or parallel, as needed by the logic.
3, For only series, it is often possible to wire a small number of two wire proximities in series. This will be on trial and error basis. Often, an interposing relay would also be needed.
One other problem I found with two wire proximity switches is that the off current is high enough to trigger the input, so I often wire the two wire proximity switches through relays as my normal policy.