The form is read by a human, so you can write anything you like, as long as it's clear. Since there's no place to explain why you want multiple entries, I would write the data for both trips in each field:
Trip #1: 4 days, Trip #2: 5 days
#1: June 1, #2: June 15
etc.
As you're probably aware (and this may not affect you), multiple entry visas are not usually granted to first-time applicants, but the worst that can happen is that they'll only grant you a single entry.