I've always signed both licences and have requested the same on revalidating my own licenses. Never a problem and keeps everything up to date. Particularly pragmatic for older pilots: it could keep them flying on a self declaration should there be a temporary hiccup with their EASA medical.
If the EASA licence was issued by another NAA then that would be a different case.
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Oh dear! Whopity is quite to raise this issue of course but here we go again - for my mind a SEPL rating is just that and it shouldn't matter to what licence it is attached. Any EASA examiner from any part of EASA land so authorised should therefore sign both of the licences rating page. When we had the UK and JAR licenses the rating page was a separate piece of paper and that could be removed and slipped into the applicable licence as and when required, simple.