Yes, you can, EASILY, it's a basic function in the FL mixer... you don't need Reason for that one... it's the function #18.
http://www.flstudio.com/htmlhelp/html/effects_trackswindow_tracktype.htm
So let's say you send the Piano1 in the mixer track #1 and piano2 in the mixer track #2 and you want to have both sent in the mixer track #3....
1 - Select the mixer track #1 in the mixer. The Track Send Enable Switch(TSES) should be pointing down... I think (going by my memory... I'm at work...)
2 - Right-click on the TSES of the mixer track #3... the arrow should light up and point up... and the one on the master should close... Now the sound of mixer track#1 is sent to the mixer track #3...
3 - Do the same with track#2...
I 'm not sure about the right or left click... but it's definitely EASY... you'll figure it out if it's not.