Why can't I use "taciti" to modify both subjects? It seems odd to repeat tacitus/tacita.
It does seem odd, and clunky. I tried "Senex tacitus tacitaque femina dormiunt," but it wasn't accepted.
The coordination is between Senex and Femina. Maybe you could try "Senex tacitus tacita feminaque dormiunt" ???
I think you want the -que at the end of the first new element that's being added.
How about "Senex tacitus feminaque tacita dormiunt" ?
Yes, you're right. On the model of Senatus Populusque Romanus.
Because this sentence is teaching you that "tacitus" is singular masculine nominative and "tacita" is singular feminine nominative. It's a two-for-one. Otherwise it would have prompted with "The quiet man and woman are sleeping."
Why « x » to sene(?) and not « s » ?
Because the adjective is "Senex, senis" ; The nominative singular is Senex.
There's no 'reason,' really, except that's what it is.