The link between the festivals of Pesach and Shavuot is not just formal and external: it is an expression of the intrinsic connection between them.
In other words, Pesach—the festival of redemption and freedom—is completed only on Shavuot, which is the festival of the giving the Torah.
Thus, Pesach without Shavuot is incomplete and lacking.
And in the same way, Shavuot needs Pesah in order to have a foundation in real life.
The two festivals are interconnected.
Whoever severs them remains with a partial entity, with only one aspect of things.
–Rabbi Adin Steinsaltz