Yes, see [1] for details. The "syphoning" exists, but it's not as bad as in a one round system where each vote on a small party is in practice lost. With two rounds, we usually have two "big" parties, flanked by two "small" parties at the extreme. People can push to one side by voting on the extreme side to extreme dissatisfaction with their closer mainline party, and up to a point it won't change the result of the second round with the two big parties. That's typically what happens, and it adds some variety with 4 large enough parties (and more very small ones of course) in the media / national discussions.

Then you have exceptional circumstances with many disgruntled people pushing for the extreme. Even then, two rounds can help. A one round system makes it very hard for the fringe to take control, but once this happen they're one of two. With a two round system only one side can go radical, with a likely result of the other side mainstream party winning.

Both sides extremes getting stronger is possible, and it's really what happened during the last presidential elections. But we got lucky and one "center" candidate went through (by a very small margin), and as expected won the second round. But as in other places we had about half people disgruntled, it's just that it was evenly split between left and right extreme parties, and only one went to second round (but the other was pretty close).

[1] https://en.wikipedia.org/wiki/Two-round_system