Output Impedance - Summing 2 Atmegatrons

[rio]

As I am still thinking of hanging 2 Atmegatrons directly to 1 FX pedal, I wanted to ask, for example, how high the output impedance of the Atmegatron is.

Can a "T-pad attenuator" be installed in a cable as described here: https://www.rane.com/note109.html?
How high must R1, R2, R3 be calculated in order not to damage the equipment and obtain good signal quality?
The input impedance of the FX device is 1 Mohm.

Thank you and Greetings!