Combine multiple ATX power supplies?
You cannot wire ATX power supplies in parallel, see this: Parallel ATX Power Supplies

You can power up multiple power supplies, and this can work for most applications where you can separate devices. The product in the discussion above can help with that. 

If you absolutely need 50 A then a 750 W plus supply may fit the bill. You could try any good quality supply like this one that delivers 54 A @ 12 V from a single supply or this one (still 750 W) that delivers 62 A @ 12 V.

Think about how you might control two regulators within a few mV - 10's mV difference in output voltage.
If you want to try this build, use a couple of LM317 regulators and try to combine and balance the output currents. It can be done by introducing balancing resistors in the outputs, but this introduces losses and degenerates voltage regulation with load.
You could start by reading articles such as this. It'll help you understand how it's done, but it's much more complicated with SMPS such as the ATX where regulation (it's called cross-regulation) is implemented across multiple voltage supplies and is typically an optically isolated feedback path. You could start reading this historical but very relevant document.
Note: It is possible to buy computer power supplies that can be paralleled, look on Ebay for redundant server power supplies. They typically have balancing built into the feedback mechanism.    

The greatest difficulty is if you want to use PS_ON* to turn your paralleled supplies on/off. The timing will be different (not by much, but enough) so that one supply will take all the load for some mS. In all probability it would shut down on overcurrent. The only way around this is to power up the supplies before connecting to the load ...which then means you need either a manual or electronic switch to connect the load. Yet more complexity!