Mistakes are made. I'm so sorry for your loss, that sounds terrible. You need to look into either rehoming one or both of your remaining rats or separating them and finding them each similar age/size companions.
In the latter case, I would still be incredibly careful introducing an adult to your older rat and take the process much slower. Research signs of aggression in rats- I can send you videos and articles, if you'd like. To introduce them, start with smell. To do so, put a barrier between them, it should be both physical and visual, and create positive associations through food.
An extra step you could take would be first having them taste different safe treats and use ones they especially like, things like: ferret malt paste, fruits- strawberries, blueberries, bananas, "human foods"- scrambled egg, rice, chicken, etc.
If that goes well, then remove the visual component of the barrier, continue to feed them together, but keep them separated. It's especially important to watch for signs of aggression at this step.
If things don't go well at any point in this process, backtrack to the previous step.
Next is to remove the barrier altogether. This should be on neutral ground, with neither of their scents present. You need to be right there with them, on guard, ready to intervene. One idea is to make a loud noise- such as knocking two pans together, and grab one while they're startled. It might be easier to go for your current rat, so long as he's used to being handled already. If not, you should work on that first. Obviously in that scenario, end their time together. I'd recommend starting all over entirely. If it does go well, you should still only have them together for a short time before returning them to separate enclosures. Continue to have them spend more and more time together before eventually, hopefully, placing the new rat into the enclosure with your current adult (still keeping an eye on them).
You should also, if you don't know this, quarantine new rats for a few days to watch for signs of illness. I can also send you articles/videos on those symptoms.
If you decide to keep both of your current rats, then I think it would be best to permanently house two different pairs, again of the same age/size.
In the latter case, I would still be incredibly careful introducing an adult to your older rat and take the process much slower. Research signs of aggression in rats- I can send you videos and articles, if you'd like. To introduce them, start with smell. To do so, put a barrier between them, it should be both physical and visual, and create positive associations through food.
An extra step you could take would be first having them taste different safe treats and use ones they especially like, things like: ferret malt paste, fruits- strawberries, blueberries, bananas, "human foods"- scrambled egg, rice, chicken, etc.
If that goes well, then remove the visual component of the barrier, continue to feed them together, but keep them separated. It's especially important to watch for signs of aggression at this step.
If things don't go well at any point in this process, backtrack to the previous step.
Next is to remove the barrier altogether. This should be on neutral ground, with neither of their scents present. You need to be right there with them, on guard, ready to intervene. One idea is to make a loud noise- such as knocking two pans together, and grab one while they're startled. It might be easier to go for your current rat, so long as he's used to being handled already. If not, you should work on that first. Obviously in that scenario, end their time together. I'd recommend starting all over entirely. If it does go well, you should still only have them together for a short time before returning them to separate enclosures. Continue to have them spend more and more time together before eventually, hopefully, placing the new rat into the enclosure with your current adult (still keeping an eye on them).
You should also, if you don't know this, quarantine new rats for a few days to watch for signs of illness. I can also send you articles/videos on those symptoms.
If you decide to keep both of your current rats, then I think it would be best to permanently house two different pairs, again of the same age/size.
