I think I finaly made it !

This is how it works:
For accounts whose inbox folder is used by more than one account, I don't launch the async getmail request anymore.
Rather, I use a function which checks them in a synchroneous way. This function is called at a rate which is set according to the smallest check interval of the concerned accounts.
If some of the accounts to be checked have a larger interval, they are skipped.
For these accounts, this implies that the actual intervals may not exactly respect the requested intervals, as they will be multiples of the smallest interval.
(Not sure to be very clear...)

Anyway, here is a beta version I would like you to test before I release it. Hope it will work for you. If it doesn't, I think I just have to

EDIT: The 0.6.4 beta has become the final 0.6.4 and the link has been disabled.