If I said,

• "On day one I give you A
• On day two I give you A and B."

Then you would have 2A and 1B.

​

Otherwise I would say

• "On Day one I give you A.
• On Day two I give you B."

Then you would have 1A and 1B.

​

>Why would anyone think that, regardless of how I write it down?

Because words have largely agreed upon meanings and if you deviate from that meaning massively people don't understand you.

If you said to someone:

"On day one, I'm giving my gf a puppy, and on day 2 I'm giving her a puppy and a ring"

Then I'm certain almost all native English speakers would understand that you have given her a puppy on day one, and then another puppy and a ring on day two.