"That woman is holding books in her hands."

Translation: Ta žena drží v rukou knihy.

[SpeculoosKoek]

And apparently 'rukou' is an example a dual preserved in Czech. The locative case when speaking about two hands (of one person is 'rukou', whereas of more people it is 'rukách')

[svrsheque]

the vestigial dual locative form "rukou" is applicable to attached limbs even when multiple people are involved. the regular plural "rukách" is also applicable either way (and to non-body situations).