My SOs MacBook Air can only do one external monitor,

The MacBook Air M4 supports two external displays now (with the lid open):

https://support.apple.com/guide/macbook-air/use-an-external-...

My SOs MacBook Air can only do one external monitor, even though it has the same specs as her work Pro.

The MacBook Pro with the non-Pro/Max chip (i.e. MacBook Pro M3) has the same limitations as the corresponding MacBook Air (i.e. MacBook Air M3).