Experts and BHG readers answer.
Rhubarb & Horseradish
For question number one, I don't have enough information to make an educated guess. I need to know more about where they are planted in your garden, what kind of weather you've been having, soil, water, etc. I don't have enough to go on.
For number two, there should be no problem growing the two plants near each other except the fact that horseradish is VERY vigorous and will take over the bed and your whole garden if you let it. You definitely need to plant it where it can be contained.