SajidJavid’s appointment as health secretary is more significant than it looks. Some Conservative MPs interpreted it as Boris Johnson installing a loyal ally to counterbalance the power of Rishi Sunak, his heir apparent as prime minister. If so, Johnson might be disappointed.

Sunak and Javid are personal friends, closer to each other than either man is to Johnson, who has few close friends in politics — a world in which so-called friends can become rivals. They are ideological soulmates: unlike Johnson, who shows no desire to call time on the spending bonanza during the pandemic, they are fiscal conservatives who believe the Tories will pay an electoral price if they do not reclaim that mantle.

Before entering politics, both Sunak and Javid went from modest beginnings to a rapid and lucrative rise in investment banking. And they are both Star Wars fans. After Javid resigned as chancellor, he tweeted about his successor: “The Force is strong in young Sunak.” Sunak, previously Javid’s number two as chief Treasury secretary, called him a Jedi Master.

Sunak and Javid also have the same instincts on Covid restrictions. With the more cautious Matt Hancock gone, the libertarian Javid will tip the inner cabinet’s balance in favour of opening up rather than locking down, which could become important this winter.

Javid’s statement this week that there is “no going back” delighted lockdown sceptic Tory MPs. But would he really defy the government’s medical and scientific advisers if they wanted to reimpose some restrictions to prevent an NHS crisis? It would be harder than Javid suggests.

Despite their common ground, the two friends would appear to be on a collision course over spending; the Treasury and Department of Health and Social Care usually are. Javid’s new empire faces huge pressures, including 5.1 million people waiting for hospital treatment; a possible “triple whammy” of Covid, flu and chest infections this winter; the timebomb of long covid; mental health provision; pay and staffing problems and preparing for the next pandemic. That’s before social care. With Johnson and Sunak deadlocked over how to fund reform, Javid and Sunak might join forces to propose a tax increase. Johnson wants the reform – likely to include a cap of about £50,000 on people’s lifetime care payments – but without the pain of tax rises the chancellor says are needed to fund it. For once, Johnson might not be able to have his cake and eat it.

Javid is well aware his Treasury successor faces a difficult government-wide spending review this autumn. His long shopping list is not the only pressing demand on Sunak.

Cabinet ministers alarmed by Johnson’s spendthrift instincts are demanding a bigger say on spending and Javid could prove a useful ally for the chancellor in such debates. During his 16 months on the backbenches after losing a power struggle with Johnson’s former adviser Dominic Cummings, Javid impressed on colleagues the danger of rising inflation and interest rates pushing up the cost of servicing the nation’s huge debt.

However, the Sunak-Javid friendship might become strained in the medium to long term because both have their eye on the top job when Johnson departs. Sunak is the front-runner for now, though is well aware the favourite rarely wins the Tory race. Javid will be back in the running if he uses the relaunch pad of his big new job well.

Although Sunak is 10 years younger than his one-time mentor, he is now the top dog. There would inevitably be other contenders, but a leadership contest run-off between Sunak and Javid would send a powerful signal about a changing Conservative Party.

In the early years of their partnership, allies of Blair and Brown vowed they would never fall out, but they did. That’s what happens when friends become rivals.  Eventually, the Sunak-Javid friendship will be tested too.