Most 2U servers use normal ATX, but instead of your typical “modern” PSU with fan on the side, it uses older style PSUs with an 80mm fan either on the back or on the front (or both for more airflow). And some cases have very ridiculous designs, in which the airflow is reversed on the PSU. You best bet, as Dutch_Master pointed out, is to buy an SFX PSU. That’s just what I did on 2x 2U server cases. Again, careful where you mount the PSU, as airflow is usually intake from the front, then it should be PSU with wires coming towards the front (to get fresh air through the side fan) and the exhaust of the PSU should go towards the back of the case.
One of my bequiet! PSUs (the SFX L POWER 500W - it’s the Power version 1, not 2) came with a metal bracket that adapted SFX to ATX, which would probably work wonders in some 2U cases (if the airflow corresponds with the mounting bracket, unfortunately in my el-cheapo cases, the airflow was inverted and I ended up having 1 fan intake in the front and 1 fan exhaust at the front in a very dust-magnet configuration, literally a vacuum cleaner - and I do have a filter on the intake, but I guess I have too much negative airflow).