It’s all about how the specific merchant processes the transaction, not what is being purchased. Most of the time, the credit card has no idea what is being purchased (item-level transaction detail does exist, but it’s rare and I’ve never heard of it actually being used in this way).
For those wondering if pay1040 lets you use multiples, I just did this last week with two cards, np. You have to submit a separate payment for each card - you can’t list multiple cards on a single payment. I just did two separate payments on two different cards for the same tax bill. And yes, the site says it’s OK to do this.
I got the Alliant 3% (first year) card with a large credit limit then they yanked the carpet out from under me by limiting the rewardable amount to $10K/month. Their notice on when the new rule kicks in was vague but my reading is that charges in February are under the old, unlimited rule. So I charged my tax payments up to my credit limit. As they used to say on SNL, I am one wild and crazy guy.