One user, two hardware phones

I don't know much about shared lines (nor their cost!) but you can assign 2 deskphones to 1 user (or you can think of it as 2 licenses to 1 user)
In this way you wouldn't need to get a shared line (which I believe is much more expensive than just getting the extra user license - not sure)

What problems can you expect by going with what I suggested? - None :]
How do you add a 2nd phone(2nd license) to a user?
Admin Portal > Users > Unassigned ext. > add unassigned ext (don't enable it)
Phone system > Phones & Devices > Unassigned > click on the ext. > Edit user > assign it to the user.

and then it is just a matter of provisioning the 2nd phone