If your main pack is woefully too small you can carry an extra pack and/or daypack---and I've done it several times in the old days. But I never strapped the extra on my chest and instead strapped it on the back or at the top of my regular pack.
The worst was in '87 during a national rainbow gathering when I had to hump out 3 packs at one time---my own big North Face external and two packs for friends who were coming in later---one with an army ALICE pack and the other a Sundog pack---all crammed atop my external. Looked like 3 bugs having sex.