Interesting question. Brings to mind an expert witness in a wrongful death suit and Perry Mason asking him to please answer "yes or no."
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Art Benstein near Baltimore

Time flies like an arrow. Fruit flies like a banana.

I'd say it's perfectly fine, especially on the rear. The rear carries some 20-30% of the braking load anyway so even without rear brakes as an extreme scenario, one will not be in great danger. What could be the cause of a potential problem? Different steel as it may have different qualities, still should have met Volvo's standards, let's hope.

I think it will be fine. I don't think the coefficient of friction for the steel will vary enough to matter. With brake pads it certainly does.
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john

I would say not a problem. On a couple of occasions I've even replaced a single rotor on one side with a new one, on the front. (This was a Taurus and a Camaro). The Camaro rotor was cracked and on the Taurus I could not remove the caliper due to a stripped hex recess in the bolt where the undamaged rotor was.

Components like shocks are should only be replaced in identical pairs because different brands and models act differently and also change during their life. Brake rotors and drums, not so much, unless you mis radically different typed, such as slotted and plain.
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1992 745, >500k km

i also would not worry about using rotors from branded different.

who knows they might even have the same source.

This is only an opinion, not facts based on knowledge.

I would not have a problem using rotors from two different manufacturers if as you say they seem identical.

What I might do is monitor pad wear a little more closely until I am satisfied they are wearing at the same rate (although I can't imagine that they would wear differently).

Randy