If you're okay with a bit of reorganization, it might be best to separate the two plurals:
"Attendees will learn practical solutions to challenges with employee benefits confronting plan sponsors."
If you're attached to the current structure, just add a dash between them. While it's not very widespread, it is accepted that one may "hyphenate two or more words when they come before a noun [so that] they modify and act as a single idea."1 In practice, this will look like:
"Attendees will learn practical solutions to employee-benefits challenges confronting plan sponsors."
The uncommon usage there may generate some confusion, so if you're looking to be safe, go with the first option and rearrange the sentence.