Can someone explain to me how loading VST instruments actually work in Dorico?

OK, so I know if I click edit instrument on the harp, and load a double bass VST, it will assign it to port 2 of channel 1, and then I can select channel 2 for the Double Bass and it activates.

However, by doing it this way, it assigns both instruments to the same mix port on the mixer, so I can no longer change the volume of the harp and double bass independently. When I add a new VST instrument on the right hand side and assign the channel to the Double Bass to fix this issue, I am incapable of hearing the Double Bass this way. Why is this? What is actually happening with the add VST section when I load another VST instrument/rack?

I clicked the cog to check where the channel and ports were and tried to assign the double bass to those to get it to sound but it isn’t doing that. I closed Dorico and reopened it as I heard this can sometimes be an issue but that hasn’t fixed it.

You should only get instruments on the same mixer slider if they are on the same VST AND they are on the same channel. Same channel on different VSTs should yield two sliders.
(Ignore Ports: keep that on 1 for everything.)