I wouldn't use two groups,, just one group, but the SL would be different.
Lets say it is skill 1, then in the agent screen you would assign skill 1, SL 1 for the pri, and SL 2 for the backups... then if the pri is busy it will goto the others,, and you only need one skill, and queue-to step in vector..
Code:
display agent-loginID 5835 Page 2 of 2
AGENT LOGINID
Direct Agent Skill:
Call Handling Preference: skill-level Local Call Preference? n
SN SL SN SL SN SL SN SL
1: 60 3 16: 31: 46: