In my experience, setting up academic jobs more than one academic year in advance is uncommon precisely for this reason: each party gets nervous that the other will back out. Thus for instance it is relatively common that one wants to accept one position and "finish out" a current position, and in my experience it is quite often acceptable to stay for one additional academic year and very rarely acceptable to stay longer than that.
I would be interested to hear more about the circumstances in which a position gets offered and accepted 1.5 to 2 years in advance. As for the ethics: formally accepting an academic position involves (i) announcing your intention to do so and (ii) signing something saying that you accept the job, with a certain start date. I do know that some positions which are funded by external grants need to be accepted conditionally on final funding of those grants (I was quite recently involved in such a situation). This is potentially awkward, as the risks involved in a conditional offer not coming to fruition are highly asymmetrical: for the employer it means that some people's time has been wasted and that certain proposed projects will not get carried out; for the employee it means that a highly trained, valued worker is out of a job for a year through no fault of their own. It may be that one has the ethical right to turn down a conditional offer for an unconditional offer under certain circumstances; I don't think you should use this an excuse, but if you honestly have doubts that the offer will become actual, then I think at some point you are entitled to take a real job rather than a virtual one. (I would be interested to hear more about this point from others.)
If you are really signing on a dotted line a commitment to a position far in the future, conditional on funding, and with an uncertain salary...well, it is not clear to me why one would sign such a contract. Barring exceptional circumstances you probably shouldn't.
I feel like the standard unit of "going through with the position you accepted" is one academic year. In other words, if you agree in writing to take a position, then ethically speaking you should keep it for at least one year (I think). Most formal contracts I am familiar with are academic year to academic year anyway: I am a tenured faculty member and I still sign a contract every August. In my department we have the understanding that a postdoc, for instance, should feel to vacate the position at the end of any given academic year if they get a better job: in fact, these usually count as success stories, including on our part. In general vacating a postdoc for a tenure-track job seems especially understandable.
All of the above makes me unsure of what the etiquette might be on a position that starts so far in the future. As you can see, I am skeptical that such an arrangement should be as ethically binding as a standard offer, but it depends on the understanding both parties had when entering the arrangement.
Added: The additional information seems helpful. Is the current postdoc at the same institution as the partner's tenure-track job and the future "soft-money" gig? I gather not. Anyway, in this case the "soft-money" job sounds more like a courtesy to the tenure track partner than anything else. Have you signed any paperwork saying that you intend to take the job? If not, I think you're totally fine not to end up taking it (however if your partner plans to stay this should be done carefully so as not to cause trouble for him/her) and they probably will not even be that broken up about it. Probably you haven't signed a contract for something so far in the future, right? If you did sign a form signifying your intent to work at a certain job, it seems to me that you need to ask if you can not take the job rather than tell. But it's hard to imagine the circumstances in which they would hold you to a future job that you no longer want.
