I'd keep Soda alone.
Seriously, I gave him about a week off after Flame was put down. Started riding him this weekend again and he has been really freaking great. Still a little spooky, but that's just his personality. That's even gotten better, now he just "looks" and gets a little tense instead of spinning and bolting. Of course that was getting better before we lost Flame. The big thing is that he was barely barn sour AT ALL.
I have a little group of 6 or so pine trees in the front of the house and a couple of lines of pine trees. I like to use the pine trees as obstacles/patterns to practice turning/weaving/etc. He usually does pretty good, but I end up using more rein than I'd like. Last night I was steering 95% w/ my legs/butt, only had to use the reins a couple of times when he got distracted. It was really really nice.
Altogether we just had a great little ride, even with the swarms of mosquitos. If he wasn't so obviously lonely I probably would be tempted to keep him alone. Oh well, if I ride him enough like this before we get the new horse maybe it'll carry over?