"Manufacturer" can indeed be both of you, but only one can receive the repairman certificate. Keep in mind that anyone can work on an experimental amateur-built aircraft. The repairman certificate only allows the sign off of the condition inspection.
...I had a partner when we built our -6A and it was registered that way "Smith/Morgan RV" but the DAR issued me the Repairman cert because I had a lot more experience. Only one of you gets it.