This is a good idea only if you know the limit to go...you shouldnt swing 40 pound dumbells the WHOLE biking routine, you should swing 5-10 pound dumbells as long as you can without totally killing your arms
A good motion to go is hold the dumbells at your side, and raise them 1 arm at a time infront of you, or to your side....then switch to the other arm....if you do choose to do this, your arms will be in some hurting the next morning. My best advice is work a completely different weight session in along with your cardio.
good luck