The "separate brokerage account" question is the tail wagging the dog. The first question you need to answer is how do you intend to manage your joint finances? There are a bunch of different ways to do this -- completely joint, completely separate, some combination of the two? What is your plan for if one person wants to FIRE and the other wants to keep working? The answer to that question is entirely different if you are planning on completely separate finances (where different accounts will be critical) vs. joint finances (where the question is more a discussion of the overall finances and timing vs. individual decisions completely independent of the other's plans/abilities). What about kids -- do you plan on having them, what if one of you wants to stay home with them for a while or takes a less-demanding/lower-paying job for the kid years? Did one of you bring in more assets or debt than the other, and if so, do you plan to keep those separate even if you combine things going forward? You need to agree on the overall plan before you can move to figuring out the best way to implenent that plan.
For us, we started out with separate accounts, because we both had savings/IRAs/etc. when we got married. We put all of our earnings into a joint account, and transferred scheduled savings from that account into a joint VTSAX account. But we also budgeted for c.$200/mo. of "fun money" that went into our individual savings accounts -- long story, but he's a spender and I'm a saver, so that kept me from losing my shit over every little stupid thing he bought. We also kept our prior savings separate (obviously necessary for the IRAs/401(k)s). That also worked, because I am naturally more a value investor and he is more growth-oriented/tech-focused/interested in individual stock-picking. So we chose a low-cost index fund for joint savings and used our existing savings to explore our individual interests. But now that we're approaching retirement, we basically count all of those individual accounts toward the communal "pot" in figuring out our retirement spending -- we're not at the point yet of figuring out what to withdraw from where, but when we do, it will likely still be from the "it's all one bucket" approach. Oh, and it also took us probably a decade to get a joint credit card; we each kept our existing ones, and just had payments withdrawn from the joint account, except when the expense was coming from our fun money.
Obviously YMMV. My SIL and BIL do the "all one pot" for everything, including pre-existing savings. My mom and stepdad, OTOH, kept entirely separate accounts and budgets until the day he died. Each of us chose different approaches, but they each worked equally well for the parties' own styles and priorities. The only question is what works for you.