They started with the hypothetical of taking a bullet for the other. (Also noted a single bullet depending on the location and whether it went fully through or is in the body and where exactly... can have high survivability.)

Ave would make it extremely dramatic then demand the doctor holds him so he can die in his arms but Ratio just suggests that in that situation he would... y' know... BE A DOCTOR.

Aventurine: "That's no fun!!! Let me die for you!!!"

Dr. Ratio: "Sometimes you really worry me."

Ratio's hypothetical however was no less dramatic, just more medically accurate.

Dr. Ratio: "If I was shot and not dead immediately then it would take a while to bleed out. I would have the time to recite my will to you so nobody can misinterpret it. Your skills with legal documents are... acceptable so perhaps there's no need for that." (high praise, acceptable=incredible)

Aventurine, disappointed it's not reciting love poems: "Am I in your will? 🥺"

Dr. Ratio: "No." (lie) "You'll be in it when we get married."

Aventurine: "OH MY LORD. NOT IF BUT WHEN??"

One of them has already bought an engagement ring. It's not Ratio. But he saw the box by accident at some point. Hence this confidence.

Bonus joke:

Aventurine: "I want you to show up to my funeral shirtless and with that handsome bust of yours."

Dr. Ratio: "I'm restricting your Tiktok access." (masktok)

Aventurine: "You can't do that!"

Dr. Ratio: "Parental controls."

Aventurine: "Noooo I'll have to watch the compilations on YouTube! You are mildly inconveniencing me! How dare you?" (smiling)