Why should Q not exceed 1/2 on a 2-stage filter?
First, I assume there is a buffer amplifier between two low-pass RC networks so that the two do not interact.  In that case, the overall transfer function is simply the product of the two 1st order LPF transfer functions:

$$\frac{V_o}{V_i} = \frac{1}{1 + sR_1C_1} \frac{1}{1 + sR_2C_2} = \frac{1}{1 + s\left(R_1C_1 + R_2C_2 \right) + s^2R_1C_1R_2C_2}$$

The standard form for a 2nd order LPF transfer function is

$$H(s) = \frac{1}{1 + \frac{1}{Q}\frac{s}{s_0} + \frac{s^2}{s^2_0}}$$

Now we can identify

$$s_0 = \frac{1}{\sqrt{R_1C_1R_2C_2}}$$

$$Q = \frac{\sqrt{R_1C_1R_2C_2}}{R_1C_1 + R_2C_2}$$

which has a maximum value of \$\frac{1}{2}\$ when \$R_1C_1 = R_2C_2\$

At a deeper level, we can write the product of two 1st order LPF transfer functions as so:

$$H(s) = \frac{1}{1 + \frac{s}{s_1}} \frac{1}{1 + \frac{s}{s_2}} = \frac{1}{1 + s\left(\frac{1}{s_1} + \frac{1}{s_2}\right) + \frac{s^2}{s_1s_2}}$$

thus

$$s_0 = \sqrt{s_1s_2}$$

and

$$Q = \frac{\sqrt{s_1s_2}}{s_1 + s_2}$$

If \$s_1\$ and \$s_2\$ are real then \$Q\le\frac{1}{2}\$.  However, if \$s_1 = s_0\$ and \$s_2 = s^*_0\$ are complex conjugates then

$$Q = \frac{|s_0|}{2\Re(s_0)}$$

which can be larger than \$\frac{1}{2}\$