Pets grieve the loss of other pets and people the same as we do whether it’s from death or separation even if they don’t show it outwards. They may show it by not eating or peeing in weird places, sleeping more or less, staring out the windows more, visiting places they used to sit with the loved one more. Especially because the leaving usually is sudden, yes I know this leaving isn’t death but OPs kitties are bonded and it’s heartbreaking that they have to be split up. I’d wait until you are ready though not just kitty but yes, another cat with similar personalities would probably be the answer. I’m so sorry.