You can't make attachment tags collaborative, but you can add an attachment tag for each recipients and make them both optional. This allows for the ability for either to upload but also allows for neither of them to upload as well. And of course if you make only one or both required, then they have to be uploaded. Unfortunately you can't have conditional logic fields that go across recipients so that's not an option.

However you could set up a structure that gets you to what you want. Bascially set up a Question & Dropdown for both recipients:  Has Attachement Been Added? Y/N

If No then the attachement tag becomes visible and mandatory for whichever recipient said NO. The other one would then be required to answer as well but can say Yes and doesn't have to attach anything.

If both recipients are at the same Order level then it wouldn't matter who went first in terms of uploading.

Hope this helps.

thank you so much for taking the time to provide such a detailed response.

Very Helpful!