I have two different GPU in my PC. When I look in the computer details page, what is shown there is a mix of the characteristics of the two boards:
https://sech.me/boinc/Amicable/show_host_detail.php?hostid=52502
It says:

Coprocessors [2] AMD AMD Radeon(TM) R5 340X (8192MB) OpenCL: 2.0

I actually have an AMD RX580 (8GB, used as "device 0") and an AMD R5 340X (2GB, used as "device 1").
My cc_config.xml file contains this:

<cc_config>
<options>
<use_all_gpus>1</use_all_gpus>
</options>
</cc_config>

Why there are not both shown? How can I correct that?

This is a known weakness in the BOINC support for GPUs as general purpose compute devices.

Although the BOINC client running on your computer is fully capable of identifying each device separately (look at the first few lines of the Event Log after restarting BOINC), the design decision taken around the year 2008 was to treat all devices as identical when reporting them to the BOINC project servers.

By 2014, it was acknowledged - see 'A Brief History of BOINC', slide 62, in the 2014 BOINC Workshop report - that there were "Reflections on software: things we need to change - Coprocessor model".

But since then, development funding for BOINC has dried up, and I see nothing to suggest that this deficiency will be addressed in the near future.

The BOINC website only shows the best GPU when more than one of the same brand - but different models - are in the machine. You cannot fix this.

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The BOINC website only shows the best GPU when more than one of the same brand - but different models - are in the machine. You cannot fix this.

It's not even the best GPU. The name is of the weakest GPU, but the amount of memory is from the stronger one.

Really, nobody is actively working on Boinc development anymore? Time to make it open source?

No full time developers, and it's been open source for many years.