Submitting an improved result before the acceptance notice of another result
Personally, when it comes to conference publications I don't worry about this type of issue.  

First, there is typically a fairly low chance that the second submission will affect the chances of the first: even in the unlikely chance that the same reviewer is looking at both papers at the same time, they are likely to understand that the second paper builds on the first: if they like the second, they'll probably suggest accepting the earlier result as well.

Second, and more importantly, conference publications are typically not the final form of a strong result.  Rather, the final form is the "extended version" journal paper that follows and supersedes the conference paper.
As such, if the second conference accepts while the first conference rejects, then there is still a clear path to publication for the results sent to the first conference: combining them together with the other results into a strong journal paper.  If the two results are related, the I think that you should be considering doing this in any case, given the typically larger scope of journal papers.

It is true that, in the worst case, it will give you one less conference publication for your C.V. If you're doing good research, though, that's not a big loss, since the work will be superseded by the journal paper in any case. Certainly it is less of a loss than not submitting to the second conference and getting rejected from the first (assuming both conferences are relatively similar in quality).