Well, I didn't keep the VIN as it was anyway because I'd prefer not to distribute power like that. But why 7.5v? USB supplies 5v, and IIRC the USB power source is connected to VIN.
Anyways, how would I go about doing 2 transistors? Should I do it like this:
5v-----Ca
RX1---Ba Cb---5v
Ea-----Bb
RX2----Eb
Where RX1 is the arduino connected to USB and RX2 is the one leeching off its data.
EDIT:
As another note, I tried connecting TX from the "host" arduino to an LED and watched it blink whenever that arduino would send data back to USB. However, if I tried connecting RX to an LED, it would just stay somewhat dim and not pulse at all whenever data would be received.