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.
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.
It is because they have a different vesting schedule from most companies. Instead of 25% of your stock annually with a 1 year cliff, they pay out 5%, 15%, 40%, 40%. So once your signing bonus is up, your shares ramp up.
