Dual verbs cannot all be formed by simply appending alif in the same way. Third-person masculine, third-person feminine and second-person dual have distinct structures.
Word / formMeaning / explanation
فَعَلَاthe two men did; third-person masculine dual.
فَعَلَتَاthe two women did; third-person feminine dual.
فَعَلْتُمَاyou two did; second-person dual for either gender.
All convey “two,” but “those two” is not “you two.” The distinction remains in the passive forms فُعِلَا، فُعِلَتَا، فُعِلْتُمَا.