Each bee hive will pollinate one wheat tile per however many ticks.
One wheat tile can only be pollinated by one hive.
If you have two hives next to each other and next to a lot of wheat tiles, then they'll cover more tiles quicker but will not cause any individual tile to grow faster.

To put it another way,
If you have 1 wheat tile and no hives, your grow speed is SLOW;
if you have 1 wheat tile and 1 hive, your grow speed is FAST;
if you have 1 wheat tile and 2 hives, your grow speed is FAST, same as 1 and 1;
if you have 2 wheat tiles and 1 hive, your grow speed is slightly slower than 2 wheat tiles and 2 hives.