Maybe late, but:

You are always welcome to board at a later station, or disembark at an earlier station. (Just let the conductor know in the early-departure case, since not all doors open at all stations.) Those are using less than the value of a full ticket.

What you can't do is disembark at a later station, or board at an earlier station -- those require using more than the value of the paid ticket, and therefore exchanging it for a higher-value ticket.

So yes, you can pay for two Seattle tickets and only use one from Olympia. The conductor probably won't even notice, as the ticket will scan as valid at any point past Seattle; s/he might not even get around to scanning anyone's tickets until after Olympia.