If you have a couple of boards then you can increase the gap. One board I'm a big believer in choosing close sizes that cover 90 percent of your wind. There is no point in having a third kite sitting in the shed depreciating for 10 percent or less kiting time.
Also it makes it a lot easier to choose which kite to put up. Windy for you 9m, crazy windy 7m.
Having more boards can really increase the bottom end of your big kite and top end of your smaller kite. This essentially increases the range between the kites.
So even if you do go just a 7 and 9 consider different board selections.
I run a 9 and 11m on a mako king, mako 140 and a skim. If light and swell just pull out the SB and go strapless. So I have close sizes and different boards which kind of contradicts the first sentence but you see what I mean.
Agreed with others stating it depends on your wind conditions, but also what you type of conditions you like. I am 78 kg, currently riding 5,7,9. I often get quite strong winds, and I am not a light wind chaser at all. 7 & 9 are two different worlds for me, if I were 60 kg I would probably go for 6 & 8.
Thank you all for your advice! I am still not sure what to choose, but I see there is nothing wrong with 7 and 9 now which is a good start The problem is that I have no local conditions, so its so hard to decide what would be the best. I even studied my kiting pictures to see what were the sizes I used the most, and its just anything between 6 and 10.
it's not a huge deal so don't stress too much about it but i would just add that different brands kites can be quite different sizes despite saying on them the same number so if you have different brands/models in quiver check up first. The problem would be where you ended up with a "small" 9m and a "large" 7m for example.
Have been really into riding in high winds I really do agree that it is better to have more small kites in quiver than bigger ones if you are into higher winds.
7/9 will work fine so will 6/9. I am 190 lbs and have 6/9/12. Unlike the old days where a standard quiver would be something like 14/12/10/8/6 these days the kites have so much range that either way it will be fine. at 60kg I would maybe consider 9/6 only because the topend of the 6 will keep you on the water longer without getting overpowered but either way you will be happy.