Generally you should have about 10x the gph through your filter as you have gallons of tank space. So on a 30 gal tank you would want 300gph of water flow through the filter. Yes you could add a second 150 and probably be good for anything, especially because you would be able to alternate changing out the filter between the two. However, this is optimal flow in the tank and not necessarily "required" to have a tank. If you keep up with cleanings and use the gravel vac with each water change, you might be able to keep up with 25" of fish and not get another filter. You would need to keep a close eye on your water quality levels, and you would do well to have at least 1 airstone in the tank to help with aeration. But other then that it can work.
one more question, the penguin 150 has an extra slot to add another filter, so the water would be filtered twice, if i put the extra filter in there, would it make a difference or would i just be wasting my money?
I don't know how much a second mechanical filter right in front of another one would really help. The 350 with it's side-by-side filters is nice, but one right on top of the other sounds like it would be a bit of a waste of money. You could maybe use the slot to put a sponge in that would be even more bio filtration for the tank which might help. So in the slot farthest away from the bio-wheel you would have the mechanical filter (either with or without activated carbon... your choice), then a sponge for bio filtration and then the bio-wheel for the ultimate bio filtration. The sponge would be a one time cost pretty much as you would only rinse it out once in a blue moon if you noticed a reduced water flow through your filter. Just make sure to get one that is made for the fish tank and not just a regular sponge that you get from the supermarket since some of those have soap or other chemicals in them.
I've noticed that the extra filter doesn't really fit and it slows down the flow so much the wheel doesn't spin and like Luniyn said, waste of money. I just have a aquaclear sponge along with their bio-bag thing with filter floss all thrown in together and it's amazing how much junk the stuff will take out. I just rinse by media and re-use.
I would suggest not to get a penguin well 150 because over time the wheel will stop turning and you will have a lot of problems with it. My 150 is junk i hate it the wheel always stops turning and i have to constantly clean the filter to get the wheel moving wish i would have bought something different.