inbound connections are at opposite ends of the building
If you ensure that you are using twisted pair cables then internal extensions are quite viable for telephone line based ADSL. It is a twisted pair to the exchange which may be a mile away, a few yards more are not going to make much difference in most cases.
One thing I did not see mentioned is that if you do go the dual line route for reliability it is essential to ensure that you have a means of detecting a line failure before both of them fail. You could for example schedule a manual cut-over every month or few in the worst case. Ideally automatic reporting should be used but that may be beyond your economic implementation.
A dual port router may provide some tools for this. If both lines are active then a special static route may allow you to route test pings out of the backup line.