First thing is, those tank lights are rarely reliable. I realize you said you're new, but you can do a search and find numerous posts about that. But, you'll figure that out as you use the rig more often.
Next... if you do have 2 black tanks, many times the handle for the 2nd tank is not located in the wet bay along side the other one. Sometimes, it's located somewhere else under the frame of the rig.. many times near the axles. Look for something like shown in the photos below: