Is it okay to use same figures in different papers?
Conferences mean very different things in different fields.
In some fields (e.g. most of pure mathematics), they are essentially just a venue for presenting/publicising your work to other researchers. In such fields, conference abstracts are not treated like full publications; there is no strong expectation of originality. So you can re-use your figures freely between conference talks/abstracts, and re-use figures from journal publications in conference talks/abstracts.
The only thing to worry about, in this case, is that if you re-use material too often, then people who see it multiple times may get bored of it, and think you are being lazy and not producing much new research. But this is unlikely if you are just re-using one or two figures, provided your talks/abstracts contain substantive other fresh material.
In other fields (e.g. much of computer science), conferences are a publication venue in their own right, on a par with journals. In such fields, there is an expectation of originality; so you should re-use figures sparingly, and usually explicitly note when you do so. Essentially, in this case, treat it as you would re-using material between papers. So it’s not necessarily a bad thing, especially in e.g. a background section, but it won’t count towards the original content that a referee will be looking for. And the re-use should be noted explicitly and prominently — e.g. “The relationship between fuzzles and woozles is shown in Figure 3 (reprinted from Jones 2014).” — unless possibly the figure is clearly just a very simple and standard illustration of background material.
There are two concerns you must address if you want to reuse a figure: "self-plagiarism" and copyright.
To avoid self-plagiarism, one simply needs to cite their previous work in which the figure was published (because some fields treat conference proceedings as publications, I'm assuming your field does as well.).
Copyright becomes more difficult and would require all of the details such as which venue you initially published in and their specific copyright policy. Here are some possible methods to avoid copyright issues:
Publish the figure first in a venue with generous copyright policy (e.g., PLoS ONE).
Modify the figure so it is different and not an exact copy.
Determine if your reuse falls under "fair use".
Obtain permission from the original publisher to reuse your figure.
I have reused figures in my own journal articles, but my employer does not allow the work to be copyrighted in the US so I only need to worry about self-plagiarism.
For most academics, copyright can be a problem. Your university likely has people who can help you. Personally, I would ask a librarian. For example, the University of Virginia's library has a resource page on how to get permission to reuse a figures.
Also, PLoS ONE discusses this topic more in a blog post on resuse and includes more details on copyright issues.
Conferences are about reaching out and sharing research with peers interested in said research.
If both conferences involve different audiences, seems fine really, and not unethical.
Unless conference organisers specifically demand that paper content be completely original.
At worse, the odd member of the audience will attend both conferences. If your papers are making different points, showing identical figures should not be an issue.