How can I justify two job changes within one year?

You have two problems, essentially. The first is getting a gatekeeper to bother to give you the chance to explain. That is, it might appear on your resume or application form that you are a job hopper; and in this market, you might not ever be brought in for an interview.

On your resume, indicate why you left each job -- not just these two, of course. Make each explanation brief and true:

Examples:
- Resigned due to family member’s health crisis requiring my attention.

- Resigned to help my mother with affairs resulting from the death of my dad.

- Employment situation was rendered untenable due to employer’s change in circumstances (or whatever you want to say caused the employer to renege on his/her word to you. Just do not make the employer out to be totally disreputable in the process if you can help it.)

- This job didn't turn out to be as described or promised. Very disappointing.

When you are interviewed, be matter-of-fact and brief about each situation. Honesty is best but do not belabor either situation. Instead, spend time pointing out your great work record at prior jobs showing your loyalty and steadfast behavior.