Putting in a dedicated power line. 2 receptacles ok?

I assume you mean a double duplex (2 port outlet)or two separate duplex outlets. I would stay with one outlet/duplex. What I did is 1 20 amp for audio and 1 20 amp for video with the grounds on opposite legs in the breaker box.

 with the grounds on opposite legs in the breaker box.

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I'm not following you. There are opposite legs in your breaker box for ground?

L1
L2
Common bus bar
(bridge bar (remove or leave) between common and ground)
Ground bus bar

Where is the second ground bus in a 220-240 single phase main?

L1 and L2 maybe.. Separate? Not a good idea that is how you get a ground loop.. Use only one dedicated 20 amp or a second BUT on the same bus, not the other.. L1 only L2 only, don't mix and match..

Regards

Why only one? Does putting two change anything?

Not really. The main benefit of a dedicated line is the elimination of connections going direct. So you lose that with the second outlet. But unless you only need two you lose that anyway with a conditioner or power strip. The main thing is if you are going to run everything through a conditioner then use one outlet and conditioner. If you will have no conditioner or things not using it then running an extra outlet (or a power strip) is the only way to go, as you want to keep it all on one line if possible.

Two is not an issue, on the same rail in the main box is. You have 2 120 vac rails. Use only one of the rails in the main.

As for the outlets you can have as many as you want, but remember the length of the wire/cable.  The further the run the higher the likely hood of an antenna affect, RFI EFI.

If you use the other leg (RAIL) you can create a ground loop. Do you understand? The difference in voltage between the TWO rails L1, L2 is usually where the noise happens. SLIGHT voltage difference, thus ground loop noise.  Because different gear is on different rails, (L1 and L2) in the main and common cabling between gear, we can create NOISE. 

I won't put up with it.. mine is dead quiet because of that alone.
Using only one side of the HOUSE MAIN... Keep the grounds and common separate in the main too.. Whole different story..  I've seen a lot of mixing the two together. Things work BUT there is a lot of voltage leakage in the circuit because of it.. Mixing the two only increases noise potential. Sometimes it's very difficult to track that type of noise down..

Regards