When we decided to swing we were both open to allowing the other to play solo, under certain circumstances. Those being that we each would have to approve of the partner, we'd have to meet them beforehand, and since our interest was primarily couples play, we would first play with the couple - as a couple - and then move on to solo play if all were open to it. This approach works well for us.
Both of us are more comfortable when playing in separate rooms or solo, but that isn't reason enough to do it without all the steps that follow up to it, as I described above.
When we began swinging, we felt it was important to know how we would feel starting on this journey together, observing each other having sex with others. You never know how you will react to that sight until it happens. We also wanted the experience together so that we could discuss it afterwards. What one of us sees and feels can be different from the other even when we are just feet away on a bed playing with a couple. We liked each other's input on how we felt the play went, what we both thought of the people and everything that surrounded swinging. It kept us feeling close and focused on helping the other fulfill a fantasy. This is why today we still start out together when playing with new people.
LM