RxSwift Shared Subscription Execution Order

How can I ensure that a subscriber to an Observable will receive the onNext event after another subscriber?

My example is the following:

let firstNameObservable = firstName.asObservable().shareReplay(1)
let lastNameObservable = lastName.asObservable().shareReplay(1)
let bioObservable = bio.asObservable().shareReplay(1)
let websiteObservable = website.asObservable().shareReplay(1)

firstNameObservable.subscribe(onNext: { [unowned self] firstName in self.accountDetails.firstName = firstName }).disposed(by: disposeBag)
lastNameObservable.subscribe(onNext: { [unowned self] lastName in self.accountDetails.lastName = lastName }).disposed(by: disposeBag)
bioObservable.subscribe(onNext: { [unowned self] bio in self.accountDetails.bio = bio }).disposed(by: disposeBag)
websiteObservable.subscribe(onNext: { [unowned self] website in self.accountDetails.website = website }).disposed(by: disposeBag)

Observable.combineLatest(firstNameObservable, lastNameObservable, bioObservable, websiteObservable)
    { [unowned self] _, _, _, _ in return self.accountDetails.validForSignUp }
    .bind(to: isValid)
    .disposed(by: disposeBag)

I would like for that last combineLatest binding to trigger after any of the 4 subscriptions above it have already executed. This is because it is only after the properties on accountDetails have been set that the validForSignUp will be accurate.

I am aware that a solution to this problem would be to make validForSignUp a Variable and observe it, but let's assume that is not possible.