I use SPA 2102 in our network. I've thousands of this phone adapters. From data sheet you can see that the Maximum Ringer Load is 3 REN.
A REN, ringer equivalence number, is the number which specifies the electrical ringing load supported from a line.
Only old telephone ringing circuits have a REN of 1. Modern phone equipments may be significantly lower than 1 because they use external power supply, digital ringer, etc. A medium value of REN is 0,1.
So you can configure a single line and connect many phones without problem. You can also configure the same number on both lines but in this way yuo have a busy line only with two concurrent calls.
We have connected up to 5 cordless to a Linksys SPA without problem.