I can see them as a shared fate for a couple reasons:

* the obvious one is Elon - both valuations are largely propped up on belief in Elon. Whenever he falters, his companies that are speculation-based (all of them) will take a hit

* Elon pitched SpaceX as an AI company. Tesla needs better AI because they keep sending signals that they won't be at L5 anytime soon, and Tesla's valuation is still very speculative[0]at least in part due to the race to L5 autonomy. i.e. Tesla will need better AI , and SpaceX is that natural fit (on paper, at least, I'm not sure SpaceX has any useful AI for any use case, let alone self-driving).

[0] Tesla's PE ratio of is still 30x massively out of line with it's actual earnings and ~30x the American automotive industry.

There are rumors that the next step is a TSLA-SPCX merger. Each company has something that Elon wants but is hard to get: Elon has super-voting controlling shares of SPCX, but TSLA is part of the S&P500.

Once a company is listed, exchange rules prohibit adding super-voting shares, it has to be done prior to listing. In order to qualify for the S&P500 a company has to have a large enough market cap and be net profitable over an entire year in the market. It seems unlikely that SPCX will qualify for that bar in the foreseeable future.

However, a merger can combine both features into one company. TSLA recently rechartered in Texas, which makes it very hard for shareholders to sue. Presumably most of the TSLA shareholders today like Elon Musk, so they would be okay with the merger, and as mentioned above Elon has full control over SPCX. Since they are in totally different markets it is hard to see what sorts of anti-trust arguments even a hostile government (e.g. Europe or Democratic state level AG's) could convince a judge of. But he does kinda need the merger to seem like something of equal companies, not an acquisition of a failing company by a successful one, so that he can keep both of the features that he wants.

Elon likes his companies financially intertwined. He convinced the Tesla board to invest in xAI to help get it off the ground, which converted to SpaceX stock after the merger, so Tesla has about $2B in SpaceX stock on its balance sheet. SpaceX is also a pretty major customer of Tesla's batteries and cyber trucks and they're partners for his terrafab scheme. Plus, Elon is likely to sell some of his personal 15% stake in Tesla if he feels it's necessary to help SpaceX, which is his favorite child. The financial ties are likely to get stronger over time.

Also, the vibe is harshed, which is actually the most important factor for these kinds of wild valuations.

There is speculation about Tesla-SpaceX merger in the future.

Investor sentiment, including talk of a potential acquisition.

Do you think the fact they are both led by the same nazi lunatic is a possible reason?