What happened to Satou at the end of the last episode of season 2? My question is, why are there two Satous at the end of the last episode of season 2; and how did the Satou with the cross-shaped scar end up masquerading as an American soldier?

To recap, Satou has his head chopped off, revives, and is then killed again and captured by American soldiers.
While being transported on the American plane, we see that there are actually two Satous: one with a cross-shaped scar on his cheek posing as an American soldier, and the real one, imprisoned with Tanaka.
There weren't 2 Satou's in there, the one with the scar was Satou in the past. He was dreaming about the time when he was still part of the American military, and captured the first demi-human known. We already knew about this in the earlier episode when the Americans were discussing the email that was sent to them by Nagai who posed as the two Americans sent by the US. Samuel T. O'Brien was the registered name of Satou and you can see the big red stamp of "Dishonorable Discharge". I could guess that he took the "SA" from Samuel, the "T" from his middle initial and the "O" from his last name to form Sato/Satou. This is just my assumption though.

The only ones in the plane are Satou and Tanaka which were placed in an astronaut suit I guess and as we can see there was a slight vibration which Satou felt before the plane exploded. So we can assume that the vibration within Satou was what caused the explosion. We still don't know how Satou did it but it may be planted inside his body but some dismissed the thought because we saw with the fight with Nagai's "flood" and him that his body was completely stabbed and hurt which would cause the possible bomb inside him to explode but if ever he found a way to put a bomb inside him within the time frame of when he was given to the Americans and the time he was transferred to the plane then that might explain it.

As for the scar that was seen in his dream, this might be because of the fact that he already died which then removed his scar completely.