I believe half siblings would generally appear like first
cousins in terms of the typical total cM length of matching
DNA segments. But the pattern should be somewhat different,
although it may be difficult to detect. With cousins there's
an equal probability of matching segments on both copies of
each chromosome. With half siblings all the matching
segments will be concentrated on one out of each pair of
chromosomes.

I guess the total cM would look like cousins, but the
individual overlapping segment lengths would be like those
of full siblings.

Elan Caspi
Belmont, CA