(Before getting into it: unless another photon propagator is being generated prior to the one in question, I would change "another" to "one". Lacking context, I will be doing that here.)
I would leave the sentence as "two fermion propagators and one photon propagator" if it's appearing a trivial number of times. If it's strongly preferable to combine them for that reason, though, I've given my initial findings below.
Considering that the nouns here are noun adjuncts (and thus function similarly to adjectives), examining patterns in that usage seems fitting. While I'm not finding anything in my Chicago style guide about this, I have found some contrasting results from some sources that (by my guess) would be applicable to your situation:
- The published research article "Intuitive Statistics by 8-month-old Infants" uses the plural form of the phrase head, opting for the phrase "a sample of four red and one white ping-pong balls" (and is agnostic about coordination order, additionally having the phrase "a sample of one red and four white ping-pong balls")
- The free online version of the textbook Probability and Bayesian Modeling, uses the singular form of the phrase head, at least when the last coordinating piece is singular, opting for the phrase "... the total number of ways of choosing two white, two red, and one black ball is the product..."
- Finally, some of the answers to the EL&L Stack Exchange question "Plural or singular noun when it refers to two things in a list?" would seem to indicate that shortening the phrase should be avoided where possible (I am not considering this question as a duplicate of that one because of the different context and the usage of cardinal numerals rather than multiple indefinite articles)
As a final note, I will say that I ran a few phrases out loud and I, personally, would probably grant the head whatever the plurality of the last coordinating element were in the situations that I would use it: "I'll have three chocolate-chip, two vanilla, and one strawberry ice-cream cone"; "I'll have three chocolate-chip, one vanilla, and two strawberry ice-cream cones"