How much do you want to pay for this? Two Mail Plus accounts is already only $8/month, which is about half of this new Duo plan already. Even if you saved an additional whole $2 (which I think would be too much of a discount tbh) that’s only $24/year, not exactly an amount worth caring about.
If they’re going to add another plan, what they should probably have is a 6 user “Mail Plus Family” plan with 100GB storage for ~$15/month. That would be pretty great for families who need email but not storage.