Each Facebook ad account comes with a pixel when the ad account is created. Pixel can be moved from an ad account to another if needed, but this is in practice quite uncommon and possibly only needed when businesses are acquired and/or responsibilities moved from a department to another.
One website can have multiple pixels and ad accounts, which is unarguably not ideal for long-term ad management as then ads are managed across two different accounts under Facebook Business Manager.
In terms of website performance, there are no negative implications of having two pixels, and as long pixels are not the same, these don’t conflict with each other either.

Nosto segments and consequently Facebook target audiences (WCA) can be used only when Nosto populates the pixel, because audiences are created based on a custom pixel event, which only Nosto can add.

A common practice is therefore following:
Advertisers / merchants typically want to keep their own ad account and consequently pixel in place when they get started with Nosto-powered Facebook ads. This requires creating a parallel ad account, and a smart practice is to create a clearly identifiable ad account with prefix or suffix -nosto in it, so that you can easily identify which ad account is tied to Nosto’s tech.

That ad account is then connected to Nosto, which enables capability to run ads through Nosto and also segment-based audiences, but only on that account.

Nothing technically prevents running all Facebook ads using the ad account & pixel tied to Nosto, but when getting started and comfortable, the described dual approach using two accounts is often the best and fastest way forward.

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