In short, Legend can not...
...however simultaneous ringing is a new feature available first in Merlin Magix R 2.2 and the newer 3.0.
If you have a CKE4 or CKE5 processor and are not using a ton of legacy equipment, I would encourage investigating the upgrade. In addition to simultaneous ring, magix 3.0 also provides numerous other friendly and useful features including auto daylight savings time adjustment, hot dial, as well as compatability with the new TDL phones.
The only caveat is that the only acceptable lines for outcalling for this feature are PRI or TIE trunks.
While I can appreciate that call setup time would make use of ls/gs lines impractical, forsight on Avaya's behalf to include BRI line pools would have been most beneficial in a couple of our applications. Maybe release 3.X will address this.