When I used 1 eternal legendary crest and 1 legendary crest, I got 16 surge points.
When I later did another run with 1 legendary crest, I got 16 surge points.
I don’t have another eternal legendary crest to try using on its own yet.
It would certainly indicate that legendary crests always grant surge points but eternal ones do not (at least, not if used in conjunction with legendary crests.