I don't have a 100. I have a 200 and 202, but the following may work on your device as well:
How to create a partial Do-Not-Disturb star code (I chose *88):
Star Codes -> Star Code Profile A -> CodeXX
*88, Partial DND, set(Sp1($Dnd),1)), set(Sp3($Dnd),1)
Save the results. Then, picking up the phone and dialing *88 will put lines 1 and 3 (Service provider 1 and 3) in DND mode. They won't ring while in this mode, and to remind you of this fact, the dial tone presented will be slow-stutter while partial DND is active. Lines 2 and 4 will continue to ring through. Of course, to cancel, just use the DND cancel code: *79.
In your case, you'd just set up one SP line in the star code above, allowing the other to continue to ring.