Are we allowed to put repeating fieldsets inside of repeating fieldsets?

The sheet I'm currently making has a system for upgrading your base whereby each structure in the base can be individually upgraded, but there's no real (rules) limit on how many structures you can have or how many upgrades each structure can have. A quick test reveals that it's possible to put a fieldset inside of another fieldset (as in, it's possible to do in the sheet testing envrionment), which would be the perfect solution to this... if it's allowed by the roll20 rules? Are there any reasons why I shouldn't do this?

Oh wait... no it doesn't quite work perfectly. The upgrades fieldset inside the structures fieldset copies the data you write into it to all other embedded repeating upgrades in all the other structures. Hm... Okay is there any way to make this work, or am I playing with fire here?

As Nice says, it's not possible to do that, but it is possible (wih a bit of work) to make it look like you have nested fieldsets, by having one fieldset that includes everything, and using CSS filters so that only what you want to be seen is displayed at a time