Approximate Gain vs Frequency Curve for an RC circuit
Am I just allowed to treat them like they're in series and use \$ C=C_1C_2\div(C_1+C_2)\$ and \$R=R_1+R_2\$ ?


No, you can't do that.

To solve this, the easiest way (IMO) is to solve for the transfer function in the s-domain:


First, find the s-domain equivalents of all the circuit elements (hint: resistors are \$R\$, capacitors are \$\frac{1}{sC}\$).
Combine R1, C2; then combine that with R2. Now you have a voltage divider, you can solve for the voltage at the junction of C1, R1, C2.
Go back to the uncombined R1, C2. You know the voltage on the left of R1, so now R1, C2 is another voltage divider. You now have transfer function of the circuit, and can easily plot the frequency response from that.