The only ascension that works on the hirling is Dealer perk.
The other perks can all work on the main character. Fortunate is the best first choice in my opinion with the hirling + dealer perk.
Epiphany set affix found at floor 600+ epic or higher difficulty . The set increase cap by 5% per rank so (5) increases all caps by 25% (except things that cannot be raised for the cap like All Sets+).
For EXP Farming, Epiphany is useful for increasing the 200% EXP gain cap to 250% because it adds 25% of 200% which is 50%. The EXP gain you can get by 6× Quest stones and 2-4 legend EXP Gain.
If you use hirlings, epiphany on both characters to get 250% EXP gain on both characters overall. 6× Quest stones on each character and some legend EXP Gain.
For Luck and Gold Find , epiphany will turn 650% into 812% at (5) and this is 1012% luck and gold find with fortunate perk. Item drops cap turns from 200% to 250% also but this turns into 350% when you use hirlings because hirlings give +100% item drops above cap.
But when you use hirlings, you will need Epiphany on both characters to get the desired effect from 650% luck to 812% and from 200% item drops to 250% which can appear as 1012% luck and 350% item drops with the fortunate perk and the 100% item drops bonus.
If you put epiphany on only one character for farming, the 25% effect is reduced to 12.5% when it comes to farm affixes due to averaging for these farm affixes. This is only exclusive to farm affixes and EXP Farming.
If you do a pure DPS build, you don't have to use epiphany on both characters so dodge 60% will turn into 75% regardless of whether you have Epiphany on 1 character or two since it's not like farm affixes where they are averaged between the two.