You can be a res without getting the SD license (that is what I did). I have the address and mail forwarding, but have yet to go to SD to get my license. My Colorado license is good for another 4 years so I am in no rush. I've known another that has done this for years without incident.
However, I do assume if you get pulled over you will be having to answer that extra question (not that you are obligated to actually answer his questions anyway, and that may be your best course of action) of why the two addresses. There is nothing that says that your PRIMARY address (what is shown on your license) has to be where you register your vehicles. Your SECONDARY address (SD address) can be a "vacation home" or "recreational property" and where you register certain vehicles. It is all perfectly legal.
Be aware, you do sign an affidavit with the state of SD stating that you do NOT have any other permanent address in any other state. In my case this is a bit of a gray area as my conceal carry permit is from Colorado and would automatically be forfeit if I am not longer a resident of Colorado. For now I still have my CO license and address... but I have a little more urgency to get up to SD and get my SD license (can get it on same day) and carry permit (must stay in state for 30 days). Alas, that will have to wait for next summer.
If you use Your Best Address (Larry is uber helpful to me) let them know that I referred you (I am box #936)
http://www.yourbestaddress.com