You can def do both and HIIT as Joe mentioned is a good way to go but you can also do more than 20min cardio in your days off from lifting if you have the energy BUT you need to EAT accordingly. To gain mass you need a minimum requirement of food and with the cardio added you need to figure what that number needs to be.
The good thing is that doing it this way for the most part will add up to lean mass gains!
Also, dont lift anymore than 3 days a week and give yourself off one day a week from any activities.
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"all i know is, when im at the beach, chicks dont ask how much i bench, they just want to know how big my arms are."  classic
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