It's not going to be much but try mining LTC with your CPU cores while your GPU's are busy mining BTC. The difficulty with LTC is so low that you may actually see some gain.
Depending on the CPU and what type of mining you are doing, CPU mining can reduce the efficiency of GPU mining. If, for example, you bought single-core CPUs for BTC mining because they are cheap, mining on that CPU may have an impact in GPU mining, especially if you are mining a coin that uses scrypt.