So sorry you are going through this.
I introduced a very cranky 15 year old (used to being alone) to an 18 year old (used to being alone) a while ago and although they will never be the best of friends, they tolerate each other well.
When I bought him home, he went in a spare room (away from the other cat) for 3 days (he was not a happy chappy). He ate in there, had a litter tray in there and I would spend 15 mins at a time in there sitting on the bed talking to him. I had a blanket in there for him and one downstairs for her and after 3 days interchanged them regularly to get them used to the smell of each other.
After 3 days, I put a piece of see through perspex over the door so the two cats could see each other but not get to each other. Did this for 2 days.
On day 6, I took the perspex down - he used the spare room as his 'base' for the following few weeks but ventured downstairs to mingle - eventually coming downstairs full time.
They both had 'their own space' and were already used to the sight and smell of each other.
Hope this helps somewhat. Can't say it'll work every time, but it did for those two.
Really hope it works out for you.