The 900 mV forward drop at 20 A per diode is the number to budget for conduction loss — at full load each diode dissipates roughly 18 W, so the TO-220 package needs a proper heatsink or forced air to stay under the 175°C junction maximum.
175°C junction — the thermal ceiling
The 175°C max junction temperature is a full-grade industrial rating. Derate the average current above 100°C case temperature per the datasheet curve — the 20 A per diode is only valid with the case held at a realistic temperature. In a 50°C ambient with a modest heatsink, expect to back off to about 12–14 A per diode to keep junction margin. The TO-220 tab is the cathode, so the common-cathode pair shares the same electrical node on the tab — useful for bolting to a shared heatsink without an insulator pad, provided the heatsink is at the cathode potential.