Should Apple be split into two different companies? One for mobile devices and one for the serious Macs?
No. Large companies that have a diversity of products do better in the long term then those that just have a one trick pony.
Generally product markets go through stages. An early phase with large growth, a mid-life phase where adoption rates normalize to the market, and later phase where the market is mature. A companies revenues are much more stable when they have a mix of products. Ideally would want one good each phase all times, but take what you can get. The high growth products tends to drive up stock price which has benefits. The mid range stuff gives steady cash flow and the later stage stuff usually leads to higher profitability (which boost stock price and provides a safety net in case hit a hiccup in other two.)
Mac OS X and iOS are tightly coupled. You can't split those two.
What folks get confused is number of dog-and-pony shows and commitments to products. The high growth stuff gets lots more dog-and-pony events.
Being a healthy, profitable company means more people will by Macs. The reverse is also true being a not so healthy and unprofitable company means less people will buy Macs. It isn't that the iOS products are there to directly boost mac sales. Not really the majority of the owners of those products don't buy Macs. That is neither required, nor necessary.
By this years end it is likely that Apple would have introduced new versions of:
Mac mini , Macbook , Macbook Pro , Mac Pro , and iMac.
a bit slightly less likely is that there will be an XServe also. That makes 5 or 6 product models introduced in 12 months. In the same 12 month span Apple will have introduced.
iPhone, iPad , iPod Touch , iPod . That is 4 product models.
[I grouped the ipod shuffle , nana , and classic together as being fair since MBP has 13" , 15" , 17" submodels too. ]
So where is the big discrepency in commitment ?
The personal computer market is mature. It is going on being 30 years old at this point. The number of larger players in the market is shrinking and they roll out updates on a more steady, less frantic basis at this point. Same thing happened in most other industries over time too. It is not a reason to split a company.