The OEM300 supply is not designed to be used in either series or parallel with another OEM300.
To be used in series, the supply output must be floating, not grounded like the OEM300. If it is floating, the voltages and wattages will sum up for 2 or more in series. i.e. two 24V 100 watt supplies with floating outputs in series will provide 200Watts at 48V.
To be used in parallel, a supply needs to have circuitry in it designed for this purpose. i.e. that it will recognize the other supply out there and form a "master/slave" relationship so that they both provide equal current or they have another cooperation scheme of some sort. The OEM300 does not have this circuitry in either the supply or the regen. So if the first 300 in parallel would fault, then the next one would fault, ditto with regen.
If more than 300W of power is required for the application, consider our OEM1000 power supply.