Problem connecting a second screen with macbook pro M1 and Dell D3100

[Simonf]

Hello,
I have a LG-34WR50QC monitor (which is a 34") and a macbook pro M1.
I try to use the picture by picture mode using a Dell D3100 but it's not working.
I can display one source (either with the a dongle or with the D3100) full width (3440 x 1440).
I can display from a dongle with a 1720x1440 resolution.
But I cannot display a source with a 1720x1440 resolution with the Dell D3100.
See:


How can I solve the issue ?

Edit: I tried with the 2 sources connected to the Dell D3100. If both are connected in HDMI I can display them but the resolution is 1920x1080 and I cannot change it to 1720x1440.

Thank you,