I only ever play split screen 2 players, we've tried alot of our own ideas.. but these ideas do depend on both players being able to handle themselves under pressure (unlike alot of the guys you find online!) Here's our favoured method to get into the 30's.
Use your colt and knife until you have 2900 each (you can do this in level3 if you get a double points etc) one of you opens the 2 left doors and gets the treench gun, the other opens the two right doors and gets the thompson (what you'll notice now is that the two most useful parts of the map are now open - this also splits up the flow of the zombies making life easier - at this point, if one of you needs reviving.. tough.. you dont have good guns at the moment and its suicide trying to get across the map to save you.. hold off the dogs and zombies until around level 8-9 (it gets hard to hold them off any longer with those guns) ofcourse if you have to you're free to make your way to the courtyard before level 9 but whoever gets their first turns the electric on, the other person then gets juggernog - you'll have quite a bit of cash now to spend on the mystry box so do so but always leave at least 1000 points each for turning the fence on in an emergancy (remember not to be stingy with your points as soon you'll be getting 30k+ points)
Anyway, you can hold off for a good few levels in this section - get speed cola when you can.. dont stay down their any later than level 16 though, you'll find it uncomfortable.. get a crawler and turn on all the teleporters and upgrade your guns (hopefully mg and ppsh and the waffle as the waffle is vital in higher rounds for shooting the bottom of the stairs) as soon as you can, now do what everyone else does and stay at the top of the stairs by the middle teleporter.. this next part isnt nessassary until the higher levels (23+) but there's no halm in doing it sooner if you're less confident - turn on the electric fence on as soon as you see a zombie running towards you - then run back to the top of the stairs where your mate will be standing, both of you should have betties by now, so place them behind you and after a few levels you should have a few betties there - this is good when you get caught having to reload - place 2 betties at each teleporter aswell (do this over a few rounds when you have crawlers) .. now around level 27-28 you'll have to adopt a new tactic, as it gets too hard to defend up there - throw your monkey as far as you can into the courtyard and run to the teleporter - waffle them as they come back if you can but otherwise wait til you cant hold them off and teleport, now this is the key to success - when you teleport run to the left teleporter but turn the fence on too (remember if you have juggernog you can run through the electric fence so dont wait for your team mate before turning it on) stay at the left teleporter and then teleport again when you have to, then run right to the right teleporter (turning the electric fence on on the way (remember we opened all the doors early on.. this is why because now its so easy to run around) stay at the right teleporter until you have to move again and then teleport.. then go to the middle teleporter.. you get where im coming from.. all you have to do is keep uniform and go left right middle left right middle left right middle and you'll be fine.. we done this with no ammo and still completed level 31 - the electric fence is so much more useful than most people give it credit for.. anyway.. it would be interesting to see how people adopt this and if they find it easier to get to higher levels.. we tried alot of different ideas - and i've read some of the ideas on here this morning and some are quite good - i havent tried any of them though, would be interesting to know how high people have got with out using glitches or the "multigun" cheats etc
the first part isnt a must, if you're not confident then dont split up.. both go right or both go left.. both cost pretty much exactly the same to get to the juggernog so its upto you