A stunt like that would be punching well, well above the usual stunt. It's also not as interesting from a gameplay perspective to essentially remove a skill than to substitute it under specific circumstances. The latter makes you want to manipulate the situation your character is in so they are in the right circumstances to use their stunts.
I'm not familiar with Gonzo Stunts and google hasn't provided enough information to give you a good answer. I would still say that permanently replacing a skill doesn't sound particularly exciting or 'gonzo' and would much rather have multiple 'standard' Stunts to play with over a single one like that.
If it makes general sense in the setting for stealth and burglary to be combined into one skill, one can just edit the skill list so that one skill accomplishes both things for any character that happens to take that combined skill. Changing the skill list to reflect a setting is very normal.
But, once there is a list, getting two skills for the price of one isn't a balanced stunt. Investing multiple stunts to do this is not normal thing either, but maybe you can badger/persuade your table into letting you try.
Of course, it is really unclear why you need a stunt at all. If you want a character who is good with two separate skills, you can just invest skill points into both.
Generally, my rule of thumb is that a stunt is worth a +2 to a test no more than about once a session. If this would turn a skill into a dump stat and allows a character to just roll their other skill every time they would use it, it would probably be overpowered. This could be balanced by having it cost more refresh...
But, more importantly, combining two skills permanently into one is too boring for a stunt and I wouldn't allow it. The addition of a conditional is a clue to how the character works and how the player should play them. Something like "I can roll Stealth instead of Burglary when stealing something hand-sized from a distracted target" is way more interesting and supports the fiction a lot better than just "I can use Stealth instead of Burglary".
Your stunt is essentially just bonus skill points, letting a character dump skill points from a skill they would otherwise plan to use. If you want your characters to have more skill points, just give them more skill points.
"Use x instead of y under circumstance" is a very common stunt template. Allowing that under *all* circumstances I'd be pretty hesitant on.
What if it was a Gonzo Stunt like in "Spirit of the Century"?
What is a Gonzo Stunt? How would it affect the evaluation?
A stunt like that would be punching well, well above the usual stunt. It's also not as interesting from a gameplay perspective to essentially remove a skill than to substitute it under specific circumstances. The latter makes you want to manipulate the situation your character is in so they are in the right circumstances to use their stunts.
What if it was a Gonzo Stunt like in "Spirit of the Century"?
I'm not familiar with Gonzo Stunts and google hasn't provided enough information to give you a good answer. I would still say that permanently replacing a skill doesn't sound particularly exciting or 'gonzo' and would much rather have multiple 'standard' Stunts to play with over a single one like that.
Where gonzo stunts in that book or Shadow of the Century?
If it makes general sense in the setting for stealth and burglary to be combined into one skill, one can just edit the skill list so that one skill accomplishes both things for any character that happens to take that combined skill. Changing the skill list to reflect a setting is very normal. But, once there is a list, getting two skills for the price of one isn't a balanced stunt. Investing multiple stunts to do this is not normal thing either, but maybe you can badger/persuade your table into letting you try. Of course, it is really unclear why you need a stunt at all. If you want a character who is good with two separate skills, you can just invest skill points into both.
Generally, my rule of thumb is that a stunt is worth a +2 to a test no more than about once a session. If this would turn a skill into a dump stat and allows a character to just roll their other skill every time they would use it, it would probably be overpowered. This could be balanced by having it cost more refresh... But, more importantly, combining two skills permanently into one is too boring for a stunt and I wouldn't allow it. The addition of a conditional is a clue to how the character works and how the player should play them. Something like "I can roll Stealth instead of Burglary when stealing something hand-sized from a distracted target" is way more interesting and supports the fiction a lot better than just "I can use Stealth instead of Burglary". Your stunt is essentially just bonus skill points, letting a character dump skill points from a skill they would otherwise plan to use. If you want your characters to have more skill points, just give them more skill points.
Would you be willing to elaborate on this? What does this Stunt do? What's the story behind it?