Spatial join features from different geodatabases

I have two geodatabases with several feature classes in both of them. I want to spatial join the feature classes in gdb1 to those in gdb2. I tried to use "arcpy.ListFeatureClasses()" function, but it needs to set the workspace to one gdb to get its list. Here is what I wrote:

import os
import pandas as pd
import arcpy

#set workspace as gdb1 and get its list
arcpy.env.workspace = r'P:JungBrownfield=2020Parcels_poly_scag.gdb'
FC_list = arcpy.ListFeatureClasses()

#set workspace as gdb2 and get its list
arcpy.env.workspace = r'P:JungBrownfield=2020Superfund_Brownfield_Database_2020.gdb'
ds_list = arcpy.ListFeatureClasses()

for i in ds_list:
    target_ft = i
    for j in FC_list:
        join_ft = j
        arcpy.SpatialJoin_analysis(target_ft, join_ft, target_ft + '_' + join_ft, 'JOIN_ONE_TO_ONE')

And I got the error –

ERROR 000732: Target Features: Dataset SF_Active_Site_Inventory does not exist or is not supported.

It seems when I changed the workspace, the feature list of the former workspace expired.

How can I solve this problem?

FC_list = [os.path.join(arcpy.env.workspace,fc) for fc in arcpy.ListFeatureClasses()]