If you are learning to read music and learning a new instrument there is an awful lot of information processing going on when you try to play a tune.
Before you even start trying to play the first note you are already remembering the time signature and the key signature as well as trying to count time. Then you look at the first note and you have to convert the note symbol and its position on the treble staff, into playable information - note duration, note pitch, whether to play the note legato, staccato etc. Then you have to relate the pitch to a specific key on the keyboard and decide which digit of your hand you are going to use to play the note.
To achieve this you have only as much time as the tune allows before you need to be playing the next note. Ideally you need to be reading ahead, processing the information on the notes to come whilst playing the current note.
The same process applies for reading a bass line.
Try playing the treble and bass together and you have more than double the amount of information processing to cope with, in the same amount of time. More than double as you now have to not just play both the left and right hands but their playing has to be synchronised to the requirements of the music.
For someone learning to play an accordion this is usually too much to process at one go. So the trick is to tackle the processing in chunks and to give yourself more time by slowing down the playing of the music. Playing slowly and maintaining time can be particularly difficult if you are familiar with the tune as your brain will tend to want to hear it played at the speed you are used to hearing the tune.
The way I have found that works for me is to enter the music I am learning into MuseScore. MuseScore is free open source music editing software. By using the computer keyboard to enter the music this provides practice in recognising the note names, because to enter a C I have to type a C.
Once the tune is entered into MuseScore, both the treble and bass lines, I can then set the tempo at which MuseScore will play the tune back. I adjust the tempo as slow as is necessary in order to play the right hand. In this way I can give myself as much time as I need to process the information. Once I can comfortably play the treble score at this speed, I increase the speed. As I become more familiar with the music the amount of mental processing required to play the piece is reduced and I can play it faster without reaching a point where I am unable to scan ahead of the notes I am playing.
Once I have the treble side at performance speed I then repeat the process just playing the left hand bass score. By the time I have the left and right hands at performance speed I have gone a long way towards having built up in muscle memory the patterns for playing the right hand and the left hand.
When I start combining the left and right hand I usually work through the score a few bars at a time initially without playback from MuseScore, just to get a feel for how the left and right hand patterns of play relate to each other. Because of the previous practice my fingers already have a feel for the notes they need to play so my concentration can be on the challenge of making the two hands play together with the right timing. When I start using MuseScore playback again I start very slowly and gradually build up the speed.
I figure that one of the things that makes a good musician is their ability to process the information they need to process in the time available. This comes from experience and practice. By slowing down the tempo you create the extra time to allow for your inexperience, in so doing you redefine the task to match your competence.
By slowing things right down you reduce the number of mistakes you make. Your muscle memory stores the movements you make. If you keep making the same mistakes you will learn those mistakes. So playing slowly and playing the right notes helps to build accurate muscle memory.
I used to struggle for weeks trying to play a practice piece at the required speed. Without MuseScore I found it really difficult to slow the playing down whilst maintaining the time, trying to slow down and keep the tune became even more information to try and process. By offloading this onto MuseScore I can create a manageable task.
Eventually you don't need MuseScore, you just play the piece unaccompanied and can then listen to your playing to improve your expression, and take responsibility for managing the timing, now that much of the information processing task has been accomplished.
I hope this helps.
