Unfortunately, two dimmers won’t work together at one light, whatever dimmer type it would be. Thus, your plan will not work. You’ll have to replace the rotary dimmer with a simple two way switch to implement dimming with Shelly. Or - much better - replace the rotary dimmer AND the two way switch with push buttons. This allows user-friendly dimming with up and down buttons.
There is only 1 rotary dimmer involved, the other switch is a simple on/off in the 2-way setup.
Replacing one rotary dimmer with some push buttons on the wall face plate of 4 switches isn't ideal - creating more work and cost for a retro fit.
So just to clarify, the Dimmer2 SW input doesn't support variable rotary dimmer switches at all?