Most professional cyclists don't want alot of upper body mass, as it just adds more weight to hold them back. They have very strong legs, but skinny arms. If your diet is proper, you can have strong cycling legs, and lift weigths to maintain your upper body mass as well.
If you do both kinds of excersise frequently, then you want to make sure you do two things to help prevent muscle loss from excessive excersise.

1. Load up on carbs prior to a bike ride. This will provide you with enough energy to help prevent your body from searching out energy by breaking down your muscle tissue. Try to eat something after your ride with a little protein to help repair your legs and respiratory muscles.

2. When you are lifting weights, do kinda the same thing, but you shouldn't need to really load up on the carbs prior to your workout as you do for cycling, as this is not cardio-vascular excersise, so it doesn't draw as many immediate calories. However, it draws tons of calories after your workout to repair the torn cells in the muscle tissue. So, what you need to do is consume lots of protein after lifting weights. Try to keep the carbs low, and protein high after lifting weights. The energy from high protein foods like steak, or whey protein shakes, will be used to repair the muscle, and excess energy will be drawn from your fat cells, to lean you out with stronger larger muscles and less body fat.

Hope this helps.