All I can say is WOW!! No doubt about it, you have the holy grail in your back yard. It's been a while since I saw such a beautiful ornamental plum tree, you've challenged me, and I'll explain why...
For all of these reasons, our challenge isn't just about pruning plum trees, it's about finding the perfect combination of beauty, design and practicality.
Here's what I'm thinking:
Design - In regards to the tree closest to the corner, I suggest you leave it for the moment (remember you can always change your mind and prune him too). He's a beautiful mature tree, and I'm sure that when he's in bloom and full of leaves he covers the entire corner, providing beauty and shade. If he produces fruits, which is great, I don't mind them growing up high at this point. Which brings me to the next tree....
Practicality - The second tree (the one that's closer to the camera), should be made the most of. Our goal here is to reduce its height so you get lots of fruits (I can't promise 200-250 pounds.... but there should be fruit), and more importantly to renew its growth and development, which is very much what pruning plum trees is all about.
When to prune? - Not now :-). I can see in the picture that both trees already have new growth and both were pruned, so what I'm about to show you relates to next winter. Just make sure you prune during dormancy or just before dormancy ends. According to the picture you sent we're too late for this season, so hold your saws for the moment and follow me....
What needs to be done?
The aim is to keep the tree's center clean so you can get better air circulation and more sunlight into it - This will help with the health of the tree. Once happy and healthy, the rest will follow. You can read more about pruning deciduous trees here.
This picture shows what I would like to achieve. our main goals are the trees' health, new growth, and better fruit (that you can also reach and pick). If you wish to get the same results from the tree in the corner, simply follow the same guidelines or, send me another picture next year - I'll be more then happy to help again.
I hope that helped
Providing general guidelines on 'how to prune' any kind of plant is important, but there are plants, and there is YOUR plant. So despite there being loads of information on this site, I also provide specific and personalized support. If something wasn't clear, or you're not sure of your plant's name, simply send me a picture with your question. Use the form below and I'll get back to you. In the mean time, see what others are asking.