Short fixing

[Farrgo]

I know that if you are soloing an aid pitch you need to set a bomber anchor for upward pull, due to the nature of system. However, if you are short fixing an aid pitch with a partner do you need to construct a bomb-proof, multi-point anchor for both upward and downward pull? It seems that if you construct a bomber anchor for downward pull with one oppositional pice, then by your partner by jugging up on the line below you, would provide a "dynamic" belay. So, if you were to rip while short fixing, your partner would take a lot of the impact off of the anchor. I'm sure it is safer to have an anchor (or two) set up to take any imaginable force, but is it good enough to have one mainly set up for downward pull and rely on your partner's body weight to make it dynamic?