You'd be wise to interpret "push back", here, as asking for a few minutes of the person's time to discuss and clarify his/her request when s(he) gives you a new assignment that is not in line with your prior, mutually agreed "Value-Adding Priority List". (You negotiated the latter during your most recent, past "Achievement Review" as outlined in #2 above. I eschew the term "Performance Review" because of its negative connotations and non-focus on value-adding.)
You also can use use your "priority list" with one manager to head off conflicting work you get from another manager when more than one person can direct your activity.
Go to the first and explain how another manager's assignment(s) is(are) making it difficult or even impossible for you to follow-through on the first manager's priorities on which you're concentrating or the deadline timing of which must be re-negotiated.
Often, your principal manager will head-off the other manager for you or get the parameters changed to something you can reasonably handle and still add your planned value.
S(he) also will do it with a lot more leverage than you have and, normally in a non-dysfunctional organization, without grief or negative fall-back for you.
If instead s(he) tells you to do both, renegotiate your previously agreed concentration level and/or timing to something mutually agreeable and reasonable for you. Then, document by e-mail your conversation and request confirmation so that there is no future misunderstanding.
However, if this happens too often, get another job ASAP because your primary boss is gutless and cannot protect you from organizational exploitation.
This is a perfect, practical and proven way to head off the overwork that many simply "put up with" in our so-called "leaner and meaner" workplace that I simply call "employee exploitation".