I'm guessing you're worried you'll miss out on things, that they'll grow closer from living together and sharing experiences and you won't be there...
But it doesn't mean that's what's going to happen. You didn't want to move in because it was too soon, and it's possible that it's the same thing for them, and that by living together they'll drive each other crazy and ruin the relationship.
Anyways, I think what I could suggest is that he goes to your place relatively often and spend the night there. This way you won't be in "their" space, he'll be in yours. That might feel better for you, since when you do go to his place, she'll probably be there too.
Or you could ask that every so often when you go to his place, you guys arrange things so that she goes out and you have the house to yourself, possibly even staying with friends for the night. Wouldn't work if it's all the time, I mean it's going to be her own place and she needs to feel welcome in it, but every so often might be an option.
Make sure to keep having enough dates to feel close to him, and try not worrying too much about the symmetry. Even if the relationships were to evolve at different rates, you can't rush yours and potentially damage it just to stay parallel to theirs. They're different relationships, so try to follow your own rhythm and you'll get there.
If the relationship becomes serious enough and you do move in with him, you will have experienced a lot of things she won't have, and vice-versa. It's not just you missing out.
Due to dating long-distance, my relationships have often been all or nothing. Either we don't see each other at all in person for months, or we live together, because going to another country costs enough money, getting a hotel there would be a nightmare.
Let me tell you I feel I missed out a lot as far as dates are concerned. No getting ready separately and meeting somewhere, looking through a crowd to find him... Instead we get ready at the same time and go there together like an old married couple.
Which is fine in an established relationship, but sometimes a relationship needs the time away from one another and then getting together again, the feeling of dates.
So, they'll spend more time together, possibly. But they'll also know each other's bad habits early on, and have arguments about milk and socks. They'll have more experiences together, but they probably won't all be positive. I think it might be a relief for him to go back to you, that he's not around 24/7, whenever you have dates, and maybe you'll spend less time together, but it could very well be better quality time.
In short, I just think you're expecting the worst and getting very worried even though it could turn out tons of different ways.