connect one of the pH meters? RX0 TX1 ??
Not and have the use of the serial monitor and easy code upload. I would put both on the I2C bus. You asked, originally, about the serial port so I didn't consider I2C. The addresses are settable so you can give each a different address. There is an I2C example on the Atlas site. Page 58 of the data sheet shows how to set the address. Learning how to use I2C is not as easy as serial, but so many things use I2C it really is worth the effort.