Your first sentence doesn't make any sense. If it's one party, eg Russia or China, then these aren't two unknown parties working together, they're one party operating at two locations. Russia and China both have the reach to operate in Cuba and China, both are devoted adversaries of the US when it comes to politics, espionage and military.

Further, it may be espionage gear that both Russia and China are now using, such that one party (Russia) is using it in Cuba, and the other party (China) is using it in China.

That is all a lot more plausible than the US severely harming their own people at their living quarters in multiple locations around the globe.

"Doesn't make any sense" is not the same thing as "doesn't match my assumptions". There is no reason to assume that one entity is at work in both of these locations, other than the one entity who we know is at work in both locations. Russia and Cuba have a long history of cooperation, but China and Cuba don't. Few in Cuba would want to piss off Russia by inviting in the Chinese for some goofy microwave project. China and Russia are also not the closest of allies. Just as the USA military-industrial complex is not a monolith, its symbionts abroad are not monoliths.

There is no reason to believe any particular thing we're told about these events. We should be extremely Bayesian, paying attention only to things we know: both phenomena affected USA diplomats, USA has routinely experimented on and otherwise disregarded the safety of those serving abroad, descriptions of symptoms are provided on the record while wide-ranging speculation is given anonymously, etc.

Very logical comment, thank you.

<joke> I assume you don't know Javascript? </joke>

I am not suggesting it is the same perpetrators, simply both cases fit the injuries consistent with microwave effects on the brain.