Of course, it is an automatic voice doing the sentences. Would the voice-double-d in "Před dvaceti" be connected to a single d (and more or less a single spoken word) or would Czech speakers actually pay regard to the separation?
You mean prorouning it as /předvaceti/? Not that is not acceptable at all. It must be /Před'dvaceti/. You must clearly produce two Ds separated by a glottal stop or even full pause when speaking slowly.