You won’t get this race condition if you let the host do this stuff.
If the user turns a knob on the UI-Editor of the plug-in the plug-in sends this change to the host. The host sends this change to the audio processor in the process call and can temporarily turn off sending automation changes until the user ends editing the knob.
The VST3-Spec does allow to set multiple parameters with the same MIDI CC.
If this doesn’t work in the hosts, than I would think that this is a bug in these hosts.