I've heard that it's best to press headset cups one at a time if you're using the block and mallet approach but I've a cheap Cyclus headset press and thought that pressing the cups together was the way to go with a headset press as the cups would be pressed parallel to each other (at least that's the way I did it for my current bike and everything's been fine).

However, I've just been watching a headset installation video on the Cane Creek website and they press the cups singly using a headset press (although they also recommend assorted cutting tools to prep the frame). What's the general view on pressing headset cups with a headset press, should I do them singley or together?

I've always done them one at a time

they'll only remain parallel to each other if the headtube faces are parallel (if you have them fully seated which is the general idea).using a block and mallet is not pressing ..rather bashing!i'd personally do them one at a time.

Always done them both together but I do have a headset press…

No need to rush. Do them individually as that way you'll always have a static surface you're pushing against.

one at a time makes it easier to keep an eye on them going in straight – doesn't really take much longer either.

I would probably do both at same time if I had a pressing tool.
I am rubber mallet man myself which I do one at a time.

There's no rush – one at a time & don't forget to put some oil on the mating surfaces

they'll only remain parallel to each other if the headtube faces are parallel

That's not really true though, is it? As long as the headtube itself is a 'true' tube, if the faces are 'off' slightly it'll look a bit unsightly, but surely the headset-headtube interface is so tight that the headtube face being slightly out wouldn't be able to 'force' the whole headset to adopt an 'off' angle within the headtube?

DrP

One at a time. Easy does it.