Nacho and Apollo catching some Z's

First birdo (on the right) my most tamest and oldest gerbil went bad. I found him having seizures, and he only lived one more week after that. Then we found out he had an abdomen tumor.

After all of that, boo started seemingly to age very rapidly an 12 days after I euthanized birdo (today) I found boo dead burried in her bedding

Now I only have one left :(

Can anyone show me what kind of setup they have for areas where they’re able to take the gerbil out of the cage, but keep them safe from escaping? Or like a separate play area from the enclosure where they can interact with them?

I have had both of my girls in a split tank for around 2 weeks now, (after quarantine in the new one ), I have swapped them on either side 1-3 times a day depending on when I get time to.
They don’t really pay attention to each other and often spend most of their time on their wheel on their enclosure side.
I have tried introducing them once after the first week and they tried to fight; thankfully I separated them and neither of them were bleeding; can’t say same for me! My finger was bleeding everywhere haha;
But since then I have attempted a introduction, but got more consistent with swapping, they use eachother wheels, sandbaths, they were originally messing up eachothers sides but now sleep in nests that have been made by the previous one; they don’t sleep next to the divider together and only occasionally check eachother out through the divider when they notice the other one there, but tend to keep to themselves.

Have I done something wrong? Should I keep trying to introduce them?

My original gerbil (squeak) sister died suddenly, they were born June 5th.
The newer gerbil (marmite) was alone when I rescued her, and had been alone for several weeks, with her only being born on July 2nd of this year.

When I first got marmite she was very relaxed and chill but now she is really excitable and squeak was the more dominant in her previous bond.

Do I try to reintroduce again today with careful watching and gloves this time?

My 3 boys all got into a very large fight where blood was drawn from all three of them, i didn’t see the fight so i do not know what the cause was nor who the main aggressor was. I’d like to be able to reintroduce them but it’s much harder to reintroduce a trio over a duo. I currently have 2 in a temporary tank and one alone while they recover. I’d like to reintroduce them but i don’t want to risk any other big fights breaking out and i’m very busy and do not have time to monitor them for hours on end and i do not want to risk any more harm.

The two i’m keeping together currently show no signs of aggression towards each other and have been sleeping and cuddling against eachother all day, the third gerbil is much more active then these two (always has been) and is very much acting his usual active self. I was planning on buying a separate tank and keeping these two together if all keeps going well. The lone gerbil is currently 1.5-2 years old and i don’t know if i should consider buying another friend for him, i don’t want to risk them not getting along and then having the same issue and being left with 3 tanks, 1 pair and 2 lone gerbils. Would it be okay for him to be a solitary gerbil if i am unable to get the trio to bond again. Their fight looked pretty severe and they’ve been prescribed Pain killers by the vets, all are showing signs of recovery.