Do you mean, where the LH plays the scale and the RH plays a third above it? (As the ones where you play in thirds with one hand are a different ball game!)
I tackled this from a few different directions and I can play all the majors, and the minors on a good day - so it's doable!
Make sure you can play the scale "as normal" in your left hand without having to think about it.
One way my teacher showed me which helped was to start the RH scale on the tonic, and then join in with the LH when the RH got to the third - feels less strange than starting on the third and gets you into the swing of things better. I also use this when I haven't played a scale in 3rds for a while and I find it's not fluent any more, it seems to help!
I am not sure how to explain it, but your 1-2-3 fingerings are together in most of the white note majors - ie where your fingering pattern is 1-2-3-1, the 1-2-3 in one hand and the 3-2-1 in the other mesh. If you're going "huh??" then ignore this, it's just a little pattern thing that I noticed.
Do them slowly to start with, better to practise slow and correct than fast with lots of mistakes, or you'll learn the mistakes.
I don't remember doing scales in thirds for grade 7?!
Practise the RH going from the third to the third, I always find it tempting to stop when the RH hits the tonic, so it's helpful to have the sound of turning around on the third in your head so you don't hear the tonic and start going the other way on automatic pilot!!
One you get going they're really not as hard or intimidating as they seem at first. Because your hands are close together, you never feel like you're trying to see what's going on and not able to (which is I think one of the things that's

about contrary motion scales). Also your fingers are following each other up the keyboard and playing all the same notes so it isn't like you're doing wildly different things, even though they're "out of sync". Once you get the feel of it then even F# major and such flow surprisingly well (in fact F# is nice because you only have a couple of white notes to worry about, instead of half and half - if you can remember that the only white notes in the scale cluster round the 3 black notes then that might help??
Hope this makes some kind of sense... if not, ignore it
