Sorry... I did some googleing and did find these sites about sidechaining:
http://logicfaq.omega-art.com/html/faq22.htm

And I did find this from old tranceaddict posts:
solitude wrote:
Short tutorial on how to 'duck' the kick with the bassline in Reason 2.5 :

1) Connect a Scream to your Kick. Turn 'Damage Control', 'Cut' and 'Body' off (you don't want to affect the sound). Now adjust the 'Master' output control of the Scream to get the kick's level right.

2) Create a Spider CV Merger/Splitter and connect the
'Auto CV Output' on the back of the Scream device to
'Split A Input' on the back of the Spider. Then connect 'Split A Output 4/Inverse' to the 'Level CV' input of the Bassline's channel in the mixer. Finally adjust the knob under the 'Level CV' to set the
"amount of ducking".

Perhaps not as good as a real sidechain, but it makes basslines a bit tighter with the kick

So I will go and try that