Roll twice and choose what suits u best in a macro

I got a player that is allowed to roll initiative with advantage and choose  the result that he likes best. That dies not necessarily need to bee the higher result. I tried to do this with roll queries, but that did not seem to work. How would I do that? 

I would probably just duplicate(copy and paste) the character's token and just roll initiative for each.  Then let the player decide which one to keep.

You can't do this with a macro, because the roll result is displayed only at the end. You can't give a player the chance to see the rolls and then choose the final result. Nor can you refer to the result of a previous macro. Vince's suggestion seems like a reasonable compromise.

Would I be able to do something like that with API-access? Maybe if I split the functionality a little like this: I write a macro that rolls initiative twice and offers clickable macros to set the initiative in the chat? The player could then simply click on the result he wants....

Yes, you could do that with the API if you were a pro subscriber. But writing API scripts is a lot more complex than creating macros. The ScriptCards API Script is probably able to do it, and learning how to use that might be the best bet. If you are willing to print the two results in chat, and let the player click a button for the one they want to you, I don't think you need the API.

I'm a cs guy, so programming is not that daunting for me :-D Could you hint me, how i can print a result and use the same result for the button in chat?

If you have the compendium, you can drag and drop, on the concerned character's sheet, the "feral instinct" feat (in the other options and features tab, or typing feral in the search bar of teh compendium) that does that automatically.