The wifi side of things is down, however there is http://gbatemp.net/threads/save-nin...e-online-servers-for-ds-and-wii-games.362717/ and it was developed using emulators so that is an option, sometimes supposedly even better than it was on the actual wifi. That said the mainline project is still down I think so you will have to set up the lot to use it.
The local aka nifi side of things has not really been tackled by much.
Some homebrew used actual local wifi of various forms, not sure what goes for emulators here, though I would not be surprised to see something. Likewise some of the ds download play stations have been emulated on the PC and if you flash your DS you can also take normal homebrew over it.
Standard wifi frequencies, not sure of the specific sub division within it and what level of frequency hopping but all pretty standard and easily found if not. However the issue was that the protocol did not use the standard header for wifi data transmission (indeed it was more headerless if what I am told is correct) and trying to get your average wifi card to broadcast what it thinks is random junk is a pain -- with the extent that the average windows OS is locked down we can not even really do some of the more interesting types of packet injection these days and that is just the OS breaking it (see raw sockets and the history thereof for that one), the header stuff is almost baked into the silicon though. We normally get to contemplate it when someone asks to punt local play over a VPN or something but in an emulator that is probably a bit easier as you do not have the data capture issue.
Specifically on the "multiplayer with 1 cart type of multiplayer" that would usually be called download play. The old wifime hack could kind of do something similar and I guess there is also the download stations, it could also be possible to do something with the wii.
If you just want to run the download play component then that can be doable. It can and does vary but if you open up the ROM and see a file called utility.bin, doubly so if it is in a folder called dwc, then that is probably the download play "ROM" (it is basically a fully functional DS ROM image so if it is not utility.bin then go looking for one of those). To what extent you might be able to play multiplayer with that I do not know but if you just want the demo/standalone options such a thing can afford then you are good.
Okay, thanks. As for the frequency issues, I have an adjustable transmitter that I can stream data to, so that's no problem. And, my question was emulating ds-to-ds multiplayer (like joining a game lobby, or Pictochat, or something of that manner.)
Pictochat is a firmware function and I believe some people have created versions of the firmware that emulators can boot. You would still be back at the nifi stuff though.
Assuming we are still talking about "single card"/download play then any "lobby" is probably a secondary thing within the game, as it would be in the normal local multiplayer. I just have to make the distinction as the download play stuff starts with a RSA signed binary being sent over wireless and loaded as a whole game on the secondary DS (the RSA was not broken, though it can be bypassed with flashme and I guess if you are emulating things then that does not matter either), the normal wifi play is pretty conventional network comms just with the headerless stuff in play and you can hack a ROM all day long to do things differently and send whatever packets you like.