This code excerpt is specific to Wisconsin, which reads similarly to some states, but not others.
a) Vertical wet vents. 1. Where 2 wall outlet fixtures are located on the same floor level with their fixture drains connecting to the same vertical drain pipe at different elevations, the lower fixture drain may be wet vented in accordance with subd. 1. a. to e. a. No other fixtures may discharge into the vertical drain pipe above or between the 2 wall outlet fixtures. Additional fixtures may discharge into the vertical drain pipe below the 2 wall outlet fixtures. b. A branch vent shall connect to the vertical drain pipe immediately above the higher fixture drain connection. c. The drain between the 2 fixtures shall be at least one pipe size larger than the upper fixture drain, but not smaller than 2º in diameter. d. Both fixture drains shall conform to sub. (9). The connection of the lower fixture drain to the vertical drain shall be considered as the vent connection. e. The higher fixture drain may not serve a water closet. Note: See ch. SPS 382 Appendix for further explanatory material.