Or is it possible to learn both without effectively learning 2 languages.
Should I learn Mexican Spanish or Spain Spanish
It depends where you live and what your goals are but they are not two different languages and, dialect and accent aside, it's much like American English vs. British English. It's easy to learn both.
They are the same language. The one thing to take note of is that in Spain vosotros is used, which is the second person plural (informal for "you" plural). This is not used in Mexico. Spanish is also spoken by people in many other countries besides Spain and Mexico.
So to answer your question, they are not two separate languages. There are just some variations. But you will find variations within any language.
HI just, have a look at this thread, this has been discussed before.
such as whether ce/ci gets pronounced as the/thi or se/si (ie, do you lisp it or not)
I am not from Spain, so I would not no if it would be offensive to call this a lisp, but the correct term for this pronunciation difference would be ceceo or seseo.
A lisp implies a speech defect that prevents or impedes pronunciation; whereas, I believe that in many areas of Spain ceceo is the norm for pronunciation and should not be considered a defect.
I'm taking the approach of trying to learn pre-colloquial Spanish, which is almost academic, and tacking on the differences on top of that. For example, even in Spain there are wide variations in pronounciation, such as whether ce/ci gets pronounced as the/thi or se/si (ie, do you lisp it or not). From Argentina, I've noticed "yo" becoming "shyo", and the 'S' being dropped between 'E' and 'C' (for example, 'escribo' and 'escuchar' become 'ecribo' and 'ecuchar'), which -completely- threw me.
There are many discussions and even some video's etc out there a search or two away where differences in accents and colloquialisms are discussed. The more you know, the more you can switch between, and it shouldn't be a huge amount more difficult than learning differences between British / American / Australian etc English, or even on smaller distances than that... a thick Scotish accent can be enough to throw me, and Scotland is but a few hours drive away, and less than an hour in the car from me you hit Dudley where English can become almost unrecognisable
As bizlaw states:
they are not two different languages
Spanish is the same for those who live in Spain as for those who live in Mexico.
Of course, there will be some differences in colloquial speech but proper Spanish is recognized in both countries.
That said, you may not need to concentrate on the "vosotros" forms of Spanish unless you plan to travel to Spain.