I know NADH is used in cellular respiration and NADPH is used in photosynthesis. What difference does the phosphate group make that the same one isn't or can't be used for both? Is there a greater reason for this separation or is it just coincidental? Why can't the two be interchanged?
The phosphate group in NADPH doesn't affect the redox abilities of the molecule, it is too far away from the part of the molecule involved in the electron transfer. What the phosphate group does is to allow enzymes to discriminate between NADH and NADPH, which allows the cell to regulate both independently.
The ratio of NAD+ to NADH inside the cell is high, while the ratio of NADP+ to NADPH is kept low. The role of NADPH is mostly anabolic reactions, where NADPH is needed as a reducing agent, the role of NADH is mostly in catabolic reactions, where NAD+ is needed as a oxidizing agent.
You'll find some more information about this in chapter 2 of "Molecular Biology of the Cell by Alberts et al.
Just to clear out some things:
As stateted above, NADH is produced in catabolic reactions and is later used in the electron transport chain to obtain energy by converting NADH back to NAD+.
NADPH is primarily produced in the oxidative part of the pentose phosphate pathway. NADPH is used in a) anabolic syntheses to produce cholesterol, fatty acids, transmittor substances and nucleotides. b) detoxifying processes as an antioxidant. NADPH is for example an essential part of CYP450 in the liver and rereduces gluthatione (one of the most potent antioxidants in nature) in order to make it active once again.