You will probably get some different types of advice because people have different experiences that color their perspectives. So, let me give you little of mine.
My wife is more sexually bohemian, I am more a monogamist at heart (wayward though I may be). Our swinging stretches over years, and has not been frequent. We have both played separately as well as together. She is totally cool about me doing it, I am VERY situational about her doing it, and she is OK with that. She has had a LOT more partners than I have, both before our marriage and since we started swinging. She understands and supports my (however unrealistic) desire to even the playing field.
She has also had quite a few more opportunities to play separately than I have. Most of these were with people I knew well, had played with together, and was totally cool with. But one, was on a business trip, back in November of last year. I had given her a free pass because it was her first business trip, she was going to the other side of the country, and I thought letting her have fun if she wanted to would be a nice gesture, as long as she came back with a really good story for me. But, I was wrong. Not about the story, but the circumstances. I will be happy to PM you if you want the details, in case they are of value, but, suffice-it-to-say, two months later, I am still very jealous over it.
So, have we played separately? yes. Was it cool? Most of the time. How much do I want to hear? Every last pornographic detail of their pleasure. How did I handle it? Again, most of the time, I was cool. I hope this answers your question and helps give you perspective.
Now, if I may offer some advice...
This is not a judgment about your arrangement in general. Nor towards you for your generosity (under the circumstances) or towards him for accepting it. But, you need your SO 100% right now, and he needs to be there for you 100%. In my mind, that can't happen while he is going elsewhere to satisfy his sexual needs. You are not comfortable with it, or you wouldn't be writing, and your stress-level can directly affect your health and your recovery.
It's noble that you don't want him to live without physical pleasure while you don't feel up to providing it. But, there is more to personal and physical intimacy than sex. Sitting with you, holding you, holding your hand, giving you massages or sharing a bath, being there while you are at the doctor or receive treatment... offering the things that make you feel supported and connected to one another, can be of immeasurable support during difficult physical and/ emotional times.
And, I know this may come across harsh in print, and I truly don't mean it to, but, he can take care of his OWN physical needs if necessary (we are, after all, "master of our domain" ). This is not too much to ask of him. It's part of what being in a relationship is about... being there in each others' time of need, sometimes at the expense of our own. However long your treatment is necessary, it is still a temporary situation, but your relationship is supposed to be permanent.
If you were my wife, I would be very receptive to you if you came to me and said, "You know I love you, and I really wanted to do this for you because I can't be what you need physically right now. But, I was wrong, this is a really bad time, this is hurting me, and I really need you to be with just me right now, even if I can't be everything I want to be for you." If it were us, I would totally understand.
I hope this helps!
Mr. Incommunicado