To me a signing bonus is a upfront payment for you to agree to join. Paying it over two years is just a regular bonus paid biweekly. Time value of money makes the lump sum a lot more valuable, especially in these inflationary times.

Sure, a single lump sum bonus might bump you in a higher tax bracket, whereas the bonus paid over 2 years may keep you in a lower bracket.

That's not really that big a deal. Sure, you might pay lower taxes if the bonus is split between two years, but the difference on tax would surely pale in comparison to typical gains in the stock market or other investment.