Can both UART and I²C be used on PIC12F1840 or PIC16F1829?
The PIC12F1840 device can use the I2C, SDA (pin5) and an SCL (pin 6) at the same time with a UART that has TX1 (pin 3) and RX1 (pin 2).
The PIC16F1829 pins RC4 and RC5 can be used for UART, you can use either SDA1/SCL1 pair which is assigned to RB4/RB6 pins or SDA2/SCL2 pair which is assigned to RB5/RB7 pins for I2C. If the device UART communication RX and TX are mapped to pins RB5 and RB7 through PPS I2C communication, you can only use SDA1/SCL1 pair which is assigned to RB4/RB6 pins.