DaisyLily my heart goes out to you... I know you said you can't stand up for yourself but what is the worst that will happen if you stood up to these two. You seem like a helpful person and don't know how to say NO that is why they will keep putting on you.
Next time they ask you to do something tell them you are busy and ask politely why they are unable to do it themself.

Is it just work load they are putting on you or is there more behind it?