I play the descant, treble and tenor recorders, harmonica, irish whistle, zither, glockenspiel, clarinet, drums, flute and am learning the violin. I teach piano, keyboard, voice and guitar. (These are all the instruments I own .. some people collect coins/stamps, I collect instruments

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When you have no other hobbies but music it's easy. I can imagine if you want to add other pursuits that take time energy and thought process, it would be difficult. Two instruments are fine when you're at school. Encourage your daughter to be organised. I'd recommend she balance the practise not equally, but devoting more time/energy etc to her priority instrument.
Each person is different. However, to me it would be an AWFUL thought to deprive someone from a musical instrument when you have the means of providing it, and encouraging it.
If she is suffering in other areas, then perhaps the guitar should be seen more as an 'outlet', a 'hobby' than true study of the instrument. I have found most kids work this out for themselves.