As POV-Ray uses most often all CPU Cores, it makes only limited sense to run
multiple instances.
There are however some cases.
For example if you have one picture that renders for long time - maybe several
days. And you want to "experiment" with another POV-Ray.

I do this often, sometimes i have 4 or 5 POV-Rays running so here are some
tipps:

1. The first thing to do is, check that in teh "Render Menu" the Render-Priority
is default on "Background".

2. Leave it on Background for the POV-Ray that is rendering for long time
anyway.
   You can set it to "Normal" if you have one POV-Ray that is rendering an qucik
Testpicture. Then this will more soon be ready.

3. The Task Manager is your friend. Sometimes it makes sense, to "free"
CPU-Cores from your system. You can do this by choosing POV-Ray "pvengine64.exe"
in the Task-Manager and the choose "Set CPU Affinity".
There you can tell which CPU-Cores are used by which instances.

This way you can reserve one core for you and windows, by just removing its
"Affinity" (remove the cross from the checkmark).

Also if you have 2 POV-Instances render long time together, you can give each of
them a specified amount of CPU Cores. Best is AB-AB-AB-AB means
Core 0 - POV - A,
Core 1 - POV - B,
etc. This way, the hyperthreading "Core" has a different program and can use the
internal execution units better.

And you avoid too many context switches like you get them if you give any
POV-Ray all the cores at the same Render Priority.

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