When diving deep or diving with a new buddy, carrying a second smaller tank, refered to as a "pony", is my recommendation.
It's not something you need another certification for. Do some reading here and consult your instructor if all isn't crystal clear.
My wife doesn't dive, so when vacation diving, I'm diving with different people.
Often they display no regard for the buddy system even if they say they will before the dive.
I have shared amazing dives with wonderful and attentive partners but that's not the norm.
Having an independent alternate gas source with you doesn't prevent you from being a good buddy but it can be very comforting when your new buddy shows no intention or maybe lacks the ability to be a decent buddy.
You are essentially a solo diver at that point. Sure, you can just surface from 35m. You're comfortable with not breathing for a 90 second accent, right?
Not me.
My emergency gas is on a short bungee necklace, 6 inches from my piehole. The tank is clipped to my left side and isn't really noticeable during a dive.
I went with a 13cu ft. It's small enough to fit in a carryon for travel and large enough to provide enough gas for a couple minutes at depth, a slow accent and a safety stop. It goes on every dive, regardless of depth or who I'm with.
I had about the same number of dives as you do now when I added it and switched to sidemount, but that's another topic all together.
Dive safe, dive comfortable!