I don't remember if your pasture leads back to the barn, or is the pasture separate from the barnyard, having the horses able to come right up to the barn when they want goes a long way to prevent them running up and down in the pasture, that's the system we have with a big run in at the back of the barn. Our horses can come up from the pasture to the barn and back out when they want.
Another thing I would suggest when you get both of the horses home, is to work them separately right from the beginning, put one horse in it's stall and take the other one out for a time, even if you don't have a lot of time, do it for a few minutes, then do it in reverse for the other one. I know a lot a people like to leave horses alone to settle in for a few days but I don't, I want to work them at the level they are trained for right away. I don't think it's fair to let them settle in to a routine and get comfortable with it, then come along and change it all on them.
RIght from the beginning let these horses know that they will have to be separated at times and learn to live with it. Saves a lot of drama later on.
Good Luck with your show. If I was taking one horse away for a day, in the beginning, I would leave the other horse in the barn until I knew how it would behave being left on it's own.
When I take my horse away to go to a show or riding with friends, I leave Sis's horse in the barn until I get back. He is very used to it now and sometime Sis will put him back out while we are gone and he is fine.
Thank you Woodhaven, those are all excellent tips!
My stalls have dutch doors that lead into a paddock, which then leads into pasture space. Initially, I thought I'd leave one in the paddock and the other in the pasture and switch them over partway through the day. Eventually, the gate can stay open (except when I need to use a dry paddock) and they can come and go as they please. There is a 10 ft overhang over the dutch doors as well, so I suspect that will be a popular spot. But there are a few other areas of natural shade throughout the pasture as well. I expect to have two water troughs in the summer. So much to plan!!!
It did occur to me that maybe I should leave the new mare (assuming I get her) in her stall for the day. It's only until 4 pm and I can leave her lots of hay. The stalls are big, roomy and very safe so she would be ok in there. That way, I don't have to worry that she escaped somehow and is who-knows-where.
And what you're saying about working with them individually makes a lot of sense. The last thing I want is for them to be reluctant to work, or reluctant to go on a trail alone. In fact, I plan on taking them both on the trails around our property individually over and over until they get really comfortable with those trails (but varying the route and trying out new trails too). The idea is that when I want to go on a trail ride with my daughter, I have a good feel for the new horse and know it won't do anything crazy. But even just bringing a horse in to groom it, do a bit of groundwork or whatever would be a good idea. I also like my horses to be easy to halter, so I go out frequently to halter them, put on sunscreen, give a treat, spray on bug spray, etc. Harley runs to me now when he sees me.