Do some planes transmit with two transponders? If not, can you explain this pair I noticed on flightradar24 just now?
http://victorspictures.com/img/s2/v59/p1920535945-5.jpg
http://victorspictures.com/img/s4/v63/p2126429438-5.jpg
http://victorspictures.com/img/s6/v135/p1982028491-5.jpg
http://victorspictures.com/img/s4/v62/p2032945698-5.jpg
Sure. Mode A/C is still required in the U.S. The requirement will be around for a while.
It is even possible(and legal as far as I know) to transmit Mode C, ES/ADS-B and UAT978 at the same time.
Thanks. I also noticed that one shows a radar for source, the other shows MLAT2. Also, nobody else seems to be on the same/similar flight path. Usually, planes follow paths occupied by a number of other aircraft.
Looks like it’s this:
flightaware.com/live/flight/N515TX
I started watching this plane because my receiver has picked up aircraft from Mexico, Oklahoma, Louisiana, and the Gulf of Mexico. So far none from Arkansas or New Mexico, although I think each is possible if conditions are perfect. This flight was heading straight to Arkansas near the edge of where my receiver’s range seems to be. I was hoping to watch it enter Arkansas air space. 
The furthest messages I seem to get are from Mexico, SW of Laredo, about 328 miles from my position. The SW corner of Arkansas is 284 miles, and the SE corner of New Mexico is 335 miles.
So this would be FR24 failing to fuse their data sources sensibly. I guess you could ask on their forums why…