Weirdly, the ability to set multiple timers varies by the device you are using.

A HomePod can set multiple timers. A Watch, iPhone, or iPad can only set one timer. There is no obvious technical reason for it. It just seems like only the HomePod team thought it was an important feature.

This becomes annoying if you have multiple devices set to respond to “hey Siri” and the wrong one picks up the request and then refuses to comply.

This used to be true, but at least the Apple Watch lets me do this all the time. I forget which OS update added the capability.

Oddly enough, in the iPadOS 15.7.1 at least if I say "Hey siri, set a timer for 20 minutes" it will say "20 minutes, starting now". Then if I say "Hey siri, set a timer for 5 minutes" it says "there's already a 20 minute timer. Replace it?"

If I say "set a timer for 20 minutes called A" it just ignores the "called A" part.

I do "Hey Siri set a timer for Foo for 5 minutes" I get a timer named Foo, and I can then set concurrent timers named Bar, Baz, etc. without replacement.

Yep, that seems to work which brings us back to the need to memorize fairly precise incantations.