groovy data grid multi dimensional mapping possible?

I use DataGridDefinitionBuilder to power the data movement from one plan type to another and use a Map variable to do some member replacements. It works great but now I have to introduce a multi dim map. My source grid columns are ordered in such a way to support what I'm trying to do:

source grid: e_123, acct123, FY24

In my Map variable I can easily insert a mapping to map any one the source values to something else:

Map myMaps = ['acct123' : 'targetAccount']

BUT if try mapping a combination of values like so it chokes:

Map myMaps = ['acct123, FY24' : 'targetAccount, FY25']