How to track equal monetary contributions by cofounders of bootstrapped startups

My cofounder and I are early in our journey with building a startup and trying to bring something to market. We're completely bootstrapped, paying for things from our day jobs' salary for now, ideally replacing that day job in the future with revenue from a successful startup.

The question has come up, how should we keep track of the money spent, so that we know one cofounder isn't putting in more money than the other.

Tracking expenses is easy, just put it in a spreadsheet (or a Notion board), however what is tricky is if there is a transfer of money between cofounders that is intended to "cover half of the costs to date".. How would this best be tracked?

I guess the absolute best situation is to have a shared bank account that money is contributed to equally, then assets are purchased from that account, but as we don't have such an account yet, we just want a way to "balance" the contributions..

Any ideas?