I'm trying to hook up the load cells from a Wii fit balance board to an esp8266 using the HX711 ADC. Ideally, a way using only one HX711 but, a hookup guide for two HX711 with two load cells apiece would be helpful as well.
In short it can be directly connected to the ADC. If you want to remove errors involved in low frequency DC signals you will need a more complex analog front end. Things such excitation methods should be consider, ratiometric measurements, differential sourcing of your load cell to improve CMRR, source loading of your sensor, common and differential mode filtering, noise, 1/f. Just top of my head. It realy depends on what accuracy are you trying to achieve. If accuracy id not your concern, than just ignore all of the above mentioned. Oh snap forgot the sensitivity of your load cell is also an important factor that must be consider. Together with you scale resolution it determines the resolution required of your ADC.
Here R#C and R#V represent constant and variable resistance inside each cell. W/B/R is one of the common color combinations for load cell wires, for +/-/C respectively. Your sensors can have different wires, of course. If that is the case, use multimeter to find the common wire. Sparkfun has convenient load sensor combinator board and very informative hookup guide for these.
Unfortunately, Wii Fit board uses 4-wire load cells that cannot be combined as above. You have two options here, either replace the cells and connect them as shown, or connect all 4 in parallel and hope that the load will not be too much for the voltage regulator.
Some other, more complex, options are to connect to the balance board via Bluetooth or hack the internal PCB to tap into I2C line. See here for related discussion.