Gas sensors for multiple gases
You are absolutely right, this sensor gives you a single output, and you don't get any hint if for example Rs/R0 = 0.9 is caused by 200ppm CH4 or 1000ppm H2.

If Rs/R0 &lt; 1, you may say that the gas is CH4,  but not CO: CO will never cause such small values within the measurement range of the sensor. But you don't know what happens for very high concentrations of CO...

If you have your CO and CH4 sensitive sensor plus a pure CH4 sensor, you can distinguish between air with CO and air with CH4, but you may not be able to identify air with both gases, because you do not know how the MQ-5 deals with mixtures. Do they sum up, or do you get the signal of the "strongest" gas only?

You definitively need sensors giving you signals sensitive to the one gas, but not the other and vice versa.