-- Last Updated: Sep-24-12 6:48 AM EST --
Are often/usually much lighter than the weight limits of third party towers and cross bars.
You say two kayaks - if you are talking about plastic 16ft or so sea kayaks you could be talking up to 60 pounds each and it would be worth reviewing the load limits on your factory bars. If you are talking a couple of short rec boats it is another matter.
There is also the mater of spacing between the cross bars, often can be made safer by going third party if you are talking about longer boats.