I think you give Johnson too much credit. He did it cos they are currently hating on each other. No other reason. He was just being a prick.
I think you haven't given him enough......he knew who ever went in there would miss the immunity challenge so he picked someone he wanted up and someone he thought he couldn't beat.......the bonus was they didn't get along and would probably not join forces
The way I saw it was thus ...
If there was no production editing here then it looked like he was given a snap decision and no time to make it. So he dumped Tim and Brenton in, on a whim, because they were fighting and he thought that it would be funny. I saw a smirk on his face as he said their names. That is how it looked to me.
However, if there was production editing, then, for all we know, he could have been given a little bit more time to think about it, possibly even with suggestions from other housemates and even BB himself. So he took his time, thought carefully, made a strategic decision (ie, he picks someone he wanted up and someone he thought he couldn't beat), says their names, in they go, and BB subsequently edits the footage to make it look like he was only given a snap decision.
I see your point, and agree putting those two people in have strategic value for him ... but he didn't know if those two would be allowed to nom, or be nommed, making it harder for him to be strategic (he would have had to simply guess one way or the other) ... and, considering what we were shown looked like a snap decision, I can't help but wonder if he would be good enough, in such a short period of time, to think logically, soundly, quickly and strategically in such a confined, pressure cooker space with everybody so close and all staring straight at him. I know I wouldn't be. But, if he could and did do that, then, kudos to him ... and, sure, I'd give him credit.