Trying to understand RLI and RLI2 better

Been reading and doing a little testing with RLI and have the following that I'm trying to understand:

I have wordlist1:
word1
word2
word3
word4

And wordlist2:
camp1
word1
word2
word3
word5

When I run this command:
./rli wordlist1 wordlist-rli wordlist2

This is what's contained in wordlist-rli
word4

What I was hoping for is this (in no particular order):
camp1
word1
word2
word3
word4
word5


Long story short is I have wordlists that I would like to merge, but also make sure they are unique.. perhaps there's a better way than using RLI