If you're adding files just to relieve allocation contention, they can be on the same drive. If you're adding files because you have IO contention on TempDB, then adding more files on the same drive will be a waste of time (more files on same drive does not improve IO throughput), hence they need to be on separate drives.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass