When two metaphors (or Metaphor Systems) are used together, there's a possibility of conflict.
Consider, for example, "As the deadline approached, the team went Balls to the Wall."
There is a conflict here between `The Future Is an Object (the deadline) Moving Toward Us` and `We Move Through Time Toward the Future` (and we'll get there faster, or in time, because we're making an extreme effort).
To Lakoff and Johnson (chapter 9) these metaphors are coherent because the visual effect is (mostly) the same whether you're moving toward an object or an object is moving toward you – in either case, the object looms 
.
I personally suspect that it also matters that there are two thoughts in the sentence:
* The problem is not much time is left, and that's not under our control. A deadline rushing toward us conveys that lack of control better than us rushing toward the deadline. In the latter case, we could just stop.
* The solution is to achieve – or get to – our goal faster. That's naturally expressed as us rushing toward the future faster.
Lakoff and Johnson refer to this as coherence between. We can permissably use these two metaphorical systems because:
1. In the Target Domain, they're "about" the same thing (the situation the team finds itself in).
2. The same is true of the Source Domain, where they're about movement through time as movement through space.
3. Neither one can be used to communicate all that's needed. One conveys lack of control over the deadline; one says you control – harder – what you can control.
See also: Just-So Stories and Coherence, Joint Coherence.