Can someone tell me why is it so dangerous to split startup company shares 50%-50% between the 2 founders?
Take it on a case by case basis, the chances are you can still succeed with a 50-50. But just as the common saying goes : "no ship can have 2 captains". Naturally, the person with the larger share say 55% will be seen as the person who has the last word, in the case of a dispute. If you also consider bringing in investors at some point and time, having a 50-50 split sends a bad signal.
It might not be, but the obvious is in the event of a dispute, you will be equally obstructive. As a practical matter, if you have investors or employees you want to incentivize, you will not have an even split for long.
Cofounders dream of being so in sync with one another that they agree 100% on issues, or agree to let the other founder handle an issue no questions asked. It's easier, seems more fair, & avoids awkward conversations.
The reality is founders will not agree on all issues. Cofounders are engaged with a startup for at least 3 years (usually 5-10) and they encounter issues they never dreamed they would, like pivoting the business model.
Would you rather be in a startup that can move forward with a decision (with the potential to either be proved wrong that it was a good decision, or have the opportunity to argue your side if their decision proved wrong), or spend time without progress because the 50/50 split cofounders can't agree on an issue.
Also, if a major decision has to be made and 1 cofounder isn't available, you're stuck again.
I'd also argue that if the cofounders sit down & list what they'll be doing, it doesn't work out to 50/50. For example, what does a non-tech cofounder of a SaaS startup do since they're not building the product? Business, product, marketing, finances, SEO, strategy, HR, sales, design, support, & some testing.