id these sf stories...

The first one sounds like the premise of Robert Sheckley's "The Native Problem" (1956, Notions: Unlimited). I'm not sure if its the one you are looking for, since like most of Sheckley's work its more of a comedy/satire with a SF backdrop than 'pure' SF. The world is also not strictly colonized, since the FTL humans consisted of only one individual, who the later arriving humans believe to be a native of the planet, despite any claims to the contrary. Nevertheless, I highly recommend all of Sheckley's work, as he is still my favorite SF author with some of the best satire ever written (imho). For the second story I don't have a clue.

Could the first one be "Time for the Stars" by Robert Heinlein? Can't remember exactly but this seems like the premise of the story or similar. Good luck on your search/research!

#1 sounds a bit like the origin story of vance astro, from marvel comics. though the rest of your synopsis suggests you're thinking of something else...

1) Far Centaurus by A.E van Vogt. Collected in http://en.wikipedia.org/wiki/Destination:_Universe!

2) Final Encounter by Harry Harrison. Collected in
http://en.wikipedia.org/wiki/Two_Tales_and_Eight_Tomorrows

My strategy was to use the tvtrope people to do the hard work:
http://tvtropes.org/pmwiki/pmwiki.php/Main/LightspeedLeapfrog
http://tvtropes.org/pmwiki/pmwiki.php/Main/HumanSubspecies

I recognize the first, but can't call up the title and author right now. I'll give it further thought. I should warn you, though, that the situation you described has been used more than once.

The second was unfamiliar to me.

I've passed on your query to an SF editorial colleague who is good at answering these questions.

Also, if you haven't already, you should look here:

http://www.sf-encyclopedia.com/

OK, I think the first one might be "Far Centaurus" by Van Vogt.

Here's a description from Wikipedia:

"Far Centaurus (1944), short story by A. E. van Vogt published in the collection Destination: Universe! (1952). A crew of Terran explorers who have been hibernating through a centuries-long voyage to Alpha Centauri discover on arrival that their technology has been radically superseded; humanity has arrived at the Alphan planet Pelham via superluminal travel long before them, and has long forgotten about them and their primitive mission (compare Comics: Guardians of the Galaxy below). The travelers must overcome their childlike naïveté to cope with the near Godlike human civilization that has evolved in their absence—a good example of the "quasimessianic ... transcendental omnipotence" with which van Vogt often furnishes his protagonists in order to generate a sense of wonder in his tales.[6]"

For the first, what leaped to my mind is "Far Centaurus," a short story by A. E. van Vogt published in the collection Destination: Universe! (1952).

The second sounds vaguely familiar but I can't hazard a guess.

#1 could be from all sorts of stories, but the likeliest is Heinlein's story of telepathic twins, one of whom stays back on Earth while the other rides off to other planets: Time for the Stars.

Not sure about #2 yet.

I can tell you they are both ideas I have toyed with in the past. The first was even outlined and partially drafted!

The second sounds like Harry Harrison's "Final Encounter". TVTropes summarises it as:

Harry Harrison's short story "Final Encounter" had a team with members of two Human Subspecies looking for nonhuman intelligence. At the end, the very promising new species, which can't even breathe the same air we do, turns out to be of Earth descent too — one group was expanding and searching clockwise around the galaxy, the other counter-clockwise.
"We are alone," Hautamaki said, looking at the massed trillions of stars. "We have closed the circle and found only ourselves. The galaxy is ours, but we are alone."

If you can't find the first, you could try posting a YASID request (Yet Another Story ID) to rec.arts.sf.written, which generally gets a series of rapid and knowledgeable replies.

I remember something like 2) - a minor plot point was that the supposed aliens had a writing system unrelated to anything known in supposedly human history, which was an argument for their actual being alien, before the biochemical analysis came in.

I'm pretty sure I read this in Asimov's SF (in German translation) sometime back in the 90s... maybe something by Poul Anderson?

Also, they have a whole page on the first plot. A bit worrying that I ended up there for both. I'd have thought the SF Encyclopedia would have been the best source.

Thanks all: Far Centaurus and Final Encounter are indeed the ones I was looking for.
The Internet again proves its wisdom again.
tvtropes sounds like an interesting resource, didn't know of it.
Was just thinking that it would have been a perfect rasfw query, didn't realise Usenet rumps still live out there.

I see that others have beat me to the answers.
Here are the collections they were published in:

Far Centaurus
http://www.isfdb.org/cgi-bin/title.cgi?41417

Final Encounter
http://www.isfdb.org/cgi-bin/title.cgi?59032

Hmph. The first storyline is exactly parallel to a short story I wrote in junior high in the early 70s. It won a grade-level English prize in my school and was 'published' in the school's literary magazine - a copy is still wedged in somewhere in my Dad's bookcase. Now I realize it must have been planted in my brain from an earlier story I had read. Please dont' tell...