Sorry, but I'm still feeling pretty dense about this. Is this what you're saying?
All two-year renewals can only be started on odd-numbered years. (The same logic would apply if we chose even-numbered years.)
If someone wants to pay for a two-year membership in 2009, they can, and will be due again in 2011.
If someone wants to pay for a two-year membership in 2010, they can't, they can only pay for one year, and will be due again in 2011. We have to hope they want to pay for two years when 2011 comes around.
I hope I'm missing something here.
Mike