I've run into this limitation before, and unfortunately, Planning does not support row-specific decimal precision. The only way to differentiate your decimal precision on a form is by account type (Currency, Non-Currency and Percentage).
What you CAN do is give one set of accounts the "Non-Currency" property, and the other set the "Currency" property. Then define different precision in the form for these account properties. This isn't a great solution because the account carries that property across all forms. So if your accounts show up in other forms, you'll have to make sure you can get your precision correct there too. Again, this isn't a great solution, but the tool is limited in this regard.
Hope this helps,