In general, I don't think you can put two regulated 24 V power supplies in parallel. Each of them will independently think they are controlling the 24 V, and will be confused by the other one also trying to control the same 24 V.
However, deeper questions arise. You have some transmitting devices or senders (flow meters--how many?), and receiving devices (controllers).
I believe 4-20 mA loops can be self-powered (the controller provides the power and the transmitter is passive), or they can be remote powered (the transmitter sends the current and the receiver is passive). There may be a configuration setting to make it work either way.
Now, if the controller can supply 500 mA, that would be enough for at least 20 loops. Do you have more than that?
If you have lots of flow senders, you might be able to power them all remotely with an external power supply, and put the controllers in passive mode so they just read the current being sent by the flow meters.
There are lots of permutations and options here, and you have not provided enough information to provide an accurate answer.
If you are in an industrial setting, you should ask a colleague like an instrument technician to advise, someone who is familiar with the specific setup you have. Unless you provide schematics, brand names and model numbers of devices, we are shooting in the dark.