Its not your fault. Please don’t blame yourself.
My two youngsters, Wilbur and Hugo, fell out six or so weeks ago.…full on fight. They now live side by side happily antagonising each other through the bars! There was a lot of territory marking behaviours upon initial separation but it has settled down somewhat now (they still rumble at each other).
Floor time for fallen out piggies needs to be separate. For Wilbur and a Hugo, their cages are side by side in an L shape, so the square area in front of the two cages is their playspace and they can remain side by side at all times (Dexter and Popcorn live in the top half of the hutch).
If they are both going to the vet at the same time, then separate carriers would be safest. You may find if you were to use one carrier for them now they’re separated, being scared means they would seek comfort from each other but the risk of them coming to blows in a confined space such as a carrier would be a big risk to take.
With Dexter and Popcorn still being a bonded pair if one needs the vet, they both go together. With Wilbur and Hugo being separated now, if one needs the vet I will probably take just that one.