designing a dual bandpass filter

If impedance is low, then a series LC makes a bandpass filter.
(a) Construct one series LC at one desired frequency.
(b) Construct another series LC at the other desired frequency.
(c) Put the filters in parallel.
(d) Each passes only its own center frequency, while stopping all other frequencies.

The reason impedance needs to be low, is because a series LC operates by having current going back and forth through it.

This filter should work between two impedances 75 ohm at input and output.

It is common with LC filters, that a suitable L:C ratio is somewhere between 100:1 and 100,000:1.

As a starting point, select L & C values which each yield approximately 75 ohms reactive impedance, at your desired center frequency. It may or may not be optimal. It illustrates how to approach the problem mathematically. You may need to adjust the ratio, to create your optimum impedance.