my 2 cents:

1. Don't do it at all, that's not serious business

2. If you really have to do something like that, use either

- pricelists

- or create different currencies

I would add a boolean or a selection field on the currency rate object.
That boolean or selection field could be used to determine if the rate is for buying or for selling.

Then, in the CRON action that retrieves the rates, i would duplicate every rate that are created (since we now need one for buying, one for selling)

Then of course, in every module that uses the currency rates, you will have to redefines the way odoo search for a currency rate, since he can't know which one you need to use :)