I have a Line 6 Spider II 150 watt head and a Marshall AV412 4x12 cab.

The Spider head outputs at 8 ohms and has 2 speaker outputs (75w each). The Marshall can take 8 ohms but only has 1 speaker input.

Is there any way I can solve this via an adapter or something?

Thanks in advance.
Not sure what kind of load using a y-cable would give to the amp (this could be very bad for your amp). I'm pretty sure they give you two outputs so you can use two cabs, or run in stereo. Read your manual (or if bought second hand, look it up on the net).
Guitars are like my babies... except without that useless baby meat...
I was thinking Y-Cable, but I don't know whether the Ohms would add up in that kind of circuit. Dammit high school physics... Come back to me, grade 12 only ended like 3 days ago.

Can anyone answer this?