You will need to install a multi-purpose base rack system on your 2004 Volvo S60. This serves as the universal platform you can then mount gear carriers to, such as the kayak carriers you will need for your two kayaks.
Both the
Thule 400XT Aero roof rack system and the
Thule 400XTR Rapid Aero roof rack system are the ideal choice for the Volvo S60. Both of these systems install and work exactly the same way, the only difference is the regular 400XT Aero rack uses the classic Thule square load bars, while the 400XTR rack uses the sleek looking Rapid Aero load bars for a more premium & refined look (which does look extremely spiffy on european model cars such as the Volvo S 60). You will use the same Thule 2106 Fit Kit clips with both of these systems too. The Thule Fit Kit clips are what fit the specific contours of vehicles for a very snug fit. There are hundreds of FitKit clips to match all kinds of vehicles.
Now for the kayak racks themselves, I do suggest going with the j-style kayak racks because of their many great features. First of all, you can attach 2 sets of j kayak racks to the base rack system load bars without having to get longer bars because the j cradle style carries each kayak in a semi-vertical fashion using up much less crossbar space than the kind of kayak racks that carry the kayak flat. If you went with a kayak rack that carries the kayak flat you would need to get the longer 58" bars. And you would not be able to use the Thule 400XTR Rapid Aero system because you can only get one specific bar length that is compatible with each vehicle (in your case you would need the RB53 53" length bars if you went with the 400XTR Rapid Aero system). The 50" length square bars are the recommended length if using the regular 400XT Aero system on your 2004 Volvo S 60. Using shorter length bars is always better when you can get away with it because you don't have to worry about the bars overhanging the roof making it a possible hazard on your head when getting in and out of your car.
The
Thule 835PRO Hull-a-Port kayak rack is the j style carrier I highly recommend. You can attach two sets of these to the base rack bars (one set on driver side and the other set on passenger side). Additionally, the cradles can instantly fold completely down when not in use too. That way you don't have to worry about any overhead clearance issues and it also makes for a very clean and aerodynamic look when not carrying the kayaks. It is very easy to place the kayaks into the cradles too. Compatible with all styles of kayaks and ideal with your 2 10 foot Wilderness kayaks.
The
Thule 835XTR Hull-a-Port kayak rack is another great option. Works exactly the same as the Thule 835PRO except that it doesn't have cradle fold-down capability.
And finally, the Thule 835Pro Hull-a-Port kayak racks transfer very quickly over to the Mercury Sable roof rack factory crossbars too. No extra adapters needed. The Thule 835PRO kayak racks come with all universal mounting hardware included for installing on any crossbars including factory racks.
The base rack systems are very user friendy and easy to install. The step by step directions make it a cinch. Usually takes between 20-40 minutes to install. The kayak racks mount super easily too and the instructions make the installation a cinch too.
Hope this helps and happy paddling!