Mount external SD-card for the secondary user
Some people wrote in 2013 that this is impossible, or at least was impossible back then. Here is what I found on a SonyMobile forum: 

"Each user gets their own [internal] space and only the owner has access to the external SD-card."

"One of the reasons for not sharing the SD card is security: FAT file system doesn't allow even for the basic Linux type permissions, thus it's impossible to secure a user's files. I agree it's a bummer but they [Google] have to go w/ the most secure of the solutions..."

(Link to the SonyMobile forum)