Hi, I know this is a common issue for beginners but is there any exercise that you can suggest in order to practice playing both of my hands together? I'm having a hard time on the first variation of "Ah, vous dirai-je maman!", and I've been practicing piano last 2 weeks. Any suggestions please? Anything will help 
The common piano technique development (Czerny) has to do with playing a finger routine, one hand at a time. However, what it does not do is to instruct those students as to how to physically effectuate this five finger routine.
And, what it definitely does not do is to have the student learn this by playing one of the most difficult pieces in the repertoire (to play correctly) which is the Mozart "Ah, vous dirai-je maman!"
Once again, Pianostreet has a marvelous search engine, which allows pianists to reference any prior question. Trust me, it is rare that any OP's question will be the first. So, go to the upper right hand corner, and work with it, even if you don't get your information on the first attempt.
Accordingly, I proffer an edited version of my prior post on piano technique, and if the OP has any questions, please do not hesitate to contact me by PM:
["With all due respect to those who have posted, I have noticed that there is no specific mention regarding any method of "key attack." This references how one/pianist strikes a particular key or a set of keys.
1) I was taught, by my late teacher Robert Weaver, that one should rest their hand, with fingers "resting" on the keys, in a standard five finger position, at "Middle C" (and a corresponding octave below).
2) That also means you are sitting erect, not stiff, but relaxed at the keyboard, with a full but relaxed arm weight.
3) Then, practice playing (super slow!) 1-5 in a very soft staccato fashion with little or no movement in either hand (super still!).
4) After one has mastered this, with one hand, the same should be repeated (slowly) with both hands. If you have to do this with two notes at a time (one to two, one to three, et al) then that is more than okay. Hand/eye co-ordination is different with every single person!
5) Then, the same modality should be effectuated with broken chords and dominant seventh and diminished arpeggios, accordingly."]