Do you leave them together when both of you go out? Work day, or shopping, dinner out, whatever?

If so, then they could stay together.

If you leave them separated still, then take him with you.

Just my thought.

When I brought "Baby Chestnut" home, he was a one pound kitten. My RB Amber was a bit put out to say the least. Sparkle took care of the baby like her own (and she had never had kittens!) and Vita kept her distance. I left Sparkle and Chestnut in my bedroom together during the day when I was at work. Eventually, I tried leaving them all in the whole apartment. I was doing this when I went out for groceries and so forth.

Came home and found Chestnut in the evergreen shrub as we entered the building. Amber swatter him off the balcony! (I had put in a pet door to the balcony, as it was the 3rd floor, so the cats never tried to jump off, too high). My neighbor saw it happen (though she didn't go down and rescue him! ) Thank goodness that evergreen was there! Chestnut was meowing up a storm, ha haa.

So Sparkle and the kitten were back in my bedroom for more weeks while I was out at work.

It happened twice more! Before finally, Chestnut was big enough to outmaneuver Amber.