Note that the middle number is irrelevant. If it goes from 0.25 to 0.75 it obviously goes through 0.5.
The way I'd do it is find all the runs where the signal is above 0.25 and discard those runs where the signal does not go above .75.
There are plenty of answers already on how to find start and end of runs (search run length encoding). One way to do it:
thresholds = [0.25, 0.75];
runs = signal > thresholds(1);
druns = diff([0 runs 0]); %start of runs are 1, end of runs are just before -1
startruns = find(druns == 1);
endruns = find(druns == -1) - 1;
From there it's simple to check which one also go above 0.75:
validruns = arrayfun(@(s, e) any(signal(s:e) > threshold(2)), startruns, endruns);
startruns = startruns(validruns);
endruns = endruns(validruns);
