In fact, I'm not entirely sure that an embassy or consulate would always agree to forward a passport on to another embassy or consulate (they might, but I don't think that they all would). And as Mr H said, would you trust someone working at an embassy or consulate to send on all the right stuff and not to have thrown it out when the application arrived as being irrelevant ("Oh, I see he's sent his application for Ruritania as well as our application: he's obviously made a mistake, so we don't need to keep all that paperwork..."). There's also the risk that the passport would never be sent anywhere, being consigned rather to the "too-hard basket" by some jobsworth or scared junior employee.

Personally, I'd either send the passport in separately for each application (especially given that there are only two of them) or else retain the services of a reliable visa agency —.who will charge, yes, but weighed against the cost of the whole trip it might not be so much.