Welcome DDS!
1. Your orchids can probably take as much humidity as you can provide them in such a dry environment. As large as a humdity tray as you can aesthetically take and then keep the water filled up right enough to keep the rocks damp - this will add as much as about 10% local humidity around your plant. Depending upon the depth of your tray you will probably need to refil 2-4 times per week. Most people don't think misting does much - it dries too quickly. If you have lots of aerial roots it wouldn't hurt to mist them in the early morning.
2. The number one cause of killing phals is overwatering. Overwatering once will not hurt a thing. Your Miltonia will need a lot more water than your phal - maybe as much as 2-3X as often. It also likes it cooler than your phal - though it likes more light. The phal should almost totally dry out between waterings and the miltonia will want to stay moist.
3. The miltonia will be good with at least that much light, the phal should get bright INDIRECT light - the leaves will easily sunburn if direct sunlight hits the leaves. If it was back 3-4 feet it might be OK - touch the leaves and make sure they are not getting warm. Air movement would help cool the leaves. The phals will take a few hours of early morning or later afternoon sun if all conditions are OK, otherwise keep to indirect.
4. While blooming you can enjoy these two plants together, but generally they are not going to be optimumally happy - if you put them where the phal wants to be the miltonia will not bloom. If you put them where the miltonia is happy the phal may suffer. If you try to find a place where each is marginally happy they "might" do OK - that means maximum sun for the phal and minimum sun for the miltonia.
5. If you use regular fertilizer made up for other houseplants - find one that is balanced - like 20-20-20. Make it up dilute, say 1/4 to 1/3 strength.
Enjoy your new orchids and keep reading and learning - BE CAREFUL - you can easily end up with a LOT of these plants if you are not careful!
mike