What you heard is true. The Spider IV HD 150 is a stereo amp and is designed to see two 8 ohm loads, one left and one right. Thus, you should connect both speaker outs on the back of your head to both inputs on the cabinet. If you only connect one side, you run the risk of burning out the power transformer on the unconnected side.  Not to mention that you would only be getting sound out of two of the four speakers in your cabinet.

Also, make sure you get speaker cables. If you use regular instrument cables to connect the head to the cabinet you will get less than desirable results.

I think I saw 2 (TWO) speaker cabs in his question. So if that is not what he really wants, I do and have searched around for the right speakers/cabs as in two (2) 4x12 cabs. Would the line 6 412VS-T and 412VS-B work with line 6 Spider IV HD150 head??? I'm guessing any TWO (2) 4x12 cabs, each rated at 8ohms would work???