Since you don't want to actually count the number of executions, simply set a global variable (globals persist across macro executions) and flip it on each run -- if you use 0 and 1 you can use the value directly in your if condition:
(You test for if 0 then do even, else do odd because the first time the macro runs the global will be undefined and the test will fail. And I've put an explicit "flip" action in each branch so you can easily see what's happening.)
Doing it like this has the added advantage that you can choose whether or not to flip the variable if your macro fails to execute for any reason. And if you ever want to start from "zero executions" again, delete Global_alternating in KM's Preferences->Variables pane.