Thank you for reaching out to ask2,extension.

The sunflowers will only affect the soil they are planted in.  If you keep them in one bed, it will not affect the other bed, but if planted too many years in a row, you will see the seeds not producing as they will eventually weaken.

To reduce the effect of sunflower toxicity, cut back, chop up and compost the plants, including their roots, in the fall (yes, the sunflower’s toxic parts decompose readily in compost bins) and rain and natural decomposition will eliminate most of the toxins left in the soil before spring. Or continue to grow sunflowers on that spot.

There are certain plants and vegetables that are resistant to the toxins.  You can find them listed on line.

I hope this helps.  You can find more information online.