OK having rummaged around to re-find the 2 fics, it is not quite as blatant copying as I first thought, so I'll give you all the urls and you can decide for yourselves.

Thank you for providing the URLs. I've just read both stories, and I don't think any plagiarism at all is involved.

I am sure there is a case of story idea copying though - firstly because I can't believe 2 people came up with this same idea,

I can. ;-) Heck, I have perpetrated a version of the same basic idea (our heroes arrive during a planetary fertility festival and get innocently caught up in it) myself. (Mardi Gras, published somewhere or other. One of Judith's zine, I think.)

It's a pretty obvious idea to occur to a writer looking for an easy way to kick some action off, especially between a pair of established characters who aren't clearly in a sexual relationship so you need to 'justify' why they do this. In fact, I'm sure I've read at least a handful along the same idea back when I read K/S, and seen the idea used in plenty of SF series.

In general, I think for there to be plagiarism in fanfic you need to find copying of actual text -- word for word lifting of sentences (possibly with names changed to fit another fandom) -- not just the same theme/basic plot. Since fanfic writers all start with the same characters, basic universe, have probably similar exposure to other books/tv shows/fairy tales and all that sort of cultural background, coincidental similarities, even striking ones, are going to be common.

and secondly because this can't be the same author at work.

Oh, yeah.

In fact, these two stories could probably be usefully studied as examples of what difference the skill of the writer make; what matters for the pleasure of the reader is NOT the idea of the story, but how that idea is carried out.

This holds on whatever aspect you want to look at. For example, Pat's story starts when it should, that is, with the situation already in progress. The back story is woven neatly into believable (not info-dumpy) dialog between A & C. The second writer wastes at least the first third of the story in setting up the scene in useless detail: we don't need to 'see' them talking about going down, discussing what they are after (since it plays no role in the story), talking about how they're going to pay for it, how much they're going to pay, teleporting, meeting and greeting and talking with the native, getting led to the festival...

Or look at characterization: Pat has Avon properly cautious/suspicious of ingesting an unknown substance, and she uses Cally's talents to investigate. They still get caught, but she offers a reason they got fooled. The second writer just has Avon and Cally acting stupidly.

Or dialog. Contrast these lines of Pat's:

"Cally." Avon half rose from the bench, a faint beading of moisture on his forehead. "Cally, that...feeling you got before, was it--?" Avon paused. His breathing sounded odd. Shallow. "No, you would not know, would you? Stupid of me. You're from Auron...reproduction by cloning."

with the second writer's:

"Cally, I think it was a bad idea to eat that fruit" Avon commented. "Do you think it was drugged?" asked Cally who was feeling pleased that she had just managed a coherent sentence. "Not, bad drugged, special fruit for this certain festival" said Avon, struggling to find the right words.

Which one 'sounds' like the Avon we saw in the series to you?

Or even just the mechanics: Pat's story has correct punctuation, format, spelling, diction. The second story... Well, there aren't commas to set off speech from the tags. She doesn't use apostrophes correctly ('we probably wont need it'.) Words are misused ('Avon followed suite') and homonyms confused ('I do to'.) The author doesn't even know she should start a new paragraph when someone new starts to speak. :-(

Pat Jacquerie was one of our best writers. The other woman....well, let's be kind and just say so far she's not.