Semi-weekly cyclic tasks

I have just picked up planner-cyclic, and it is working well for the
weekly tasks I specify in .diary.cyclic-tasks. 

However, I can't figure out how to specify tasks that are repeated
every second week.  A simple and sufficient solution would be to use
week numbers, and say "do this on Tuesday every even-numbered
week". However, it seems week numbers are not supported by the diary
date formats. (My experience with diary is minimal, sorry.)

Surely semi-weekly, semi-annual, &c, tasks (events) are quite
common. Has anyone come up with a way to specify them, in diary
format?

Best,

Johan.