It is happening while rig is on for several days. The fuse is old style one time use.
There are several reasons what could be:
1. Your fuse is bad quality, or it is wrong rating
2. There was a power outage, or several power outages at short time. Current surge at both PSU powerups exceed the current rating and blows the fuse.
3. Load is more than what you think
4. Your PSU is going bad.
For point 1. It is possible your fuses are bad rating. Verify that it really is 20A fuse.
For point 2. When you power on the PSUs, especially 2 of them at the same time, there is massive current surge (2-5x more than rated) for very short period of time. In my opinion this is the most likely reason for your issues, that there was outage of power and then it blew up. Check if your fuse has a SNAIL marking on it (slow-blowing fuse, or delayed action fuse, what ever is the name, there will be a snail or letter T (timed) or letter L (lag) next to 20A rating)
3. Maybe your rigs lose some power optimizations after some time and run the cards overvolted. Very unlikely considering you would need more than 2.2KW of load to blow the fuse.
4. Very unlikely considering you experience this every few months