Kyle : Are you taking on this project as the Knight in Shining DJ Armor or has the bride to be solicited your real help or has asked the deadliest question of all times, the off-handed insincere "What do you think?"
My recommendation would be to politically stay completely out of and from under this. She knows in her heart of hearts what she feels and further more what should be done and how. The situation is what it is, so she has to suck it up and select from a list of possible solutions:
1) one dance starting with the real Dad, then half way through the Step Dad cuts in and finishes the job (that kinda says it all, doesn't it?);
2) two separate songs, two separate dances, but now the audience sits through 10 minutes of uncomfortably instead of 5 minutes as in example 1);
3) don't do any formal father/step father-daughter dances but during the evening she dances with both at two separate points in time and have the photographer shoot it unannounced;
4) Fred's idea is novel and perhaps a great sign of solidarity in the brides families however that's 15 minutes of bride's family only while the groom's family and all the guests are forced to sit and observe this pseudo-healing that probably will remain revert unchanged just as it was prior to the reception.
If there's help needed in them finding songs, have them try this page where they can listen to about 50 choices on their own time frame perhaps even with the people involved:
http://www.imadj.com/w_wedding/parent_dances.shtml
Regardless of their decisions, good luck with this prickly situation from your stand point.