I believe the intent of your examples is to show contrast
single yet attached
alone yet accompanied
and "still" can be used to show this contrast also, however "still" implies a continuation of state, so logically it would be
attached yet still single
accompanied yet still alone
since in both cases the assumed initial sites would be "single" and "alone", and as has been pointed out in the comments, your use of "yet" is similar to "but"
attached but (still) single
accompanied but (still) alone
both work, however
attached still single
accompanied still alone
do not. So, no, you cannot simply replace "yet" with "still".