Rules as written in Ironsworn and Starforged, you always use 1d10 for both challenge dice. Some third-party expansions, however, tweak this mechanic.
Use the Set action roll challenge dice sides command when you want to pick different action roll challenge dice. You will be prompted for which die to use for the first and second action roll challenge dice independently.
This is a "campaign local setting" so it will only apply for the current campaign and will not be synced.
Not all Iron Vault features have been validated against non-standard challenge dice settings. If you notice anything misbehaving when you use non-standard challenge dice, please file a bug report at https://github.com/iron-vault-plugin/iron-vault.