dfh is just a smart name for SFH when there is a little amount of trx in the cell, right ?
but doing sfh in a 2trx cell means the bcch freq is not hopping… Applying the sfh would “only” bring better rxlev during a call on 2nd trx. Which is of course a good thing anyway since most calls will be allocated on 2nd trx 🙂
Now, going even further : it means you need to have the SDCCH on the 2nd TRX too, to fully benefit from FH during the call setup. And since you must have a SDCCH ts on 1st TRX too, it means you need 2 SDCCH ts on such cell. (not really a problem) and by default, it is the sdcch on the last trx that is used preferably.