Granting access to one db to users/roles of another
USE [database1]
GRANT EXECUTE ON [database2].[dbo].[CUSTOM_PROCEDURE] TO [applicationrole1]

OR

USE [database2]
GRANT EXECUTE ON [dbo].[CUSTOM_PROCEDURE] TO [database1].[dbo].[applicationrole1]

INSERT INTO Database2.sys.database_principals
SELECT * FROM Database1.sys.database_principals
WHERE [type] = 'S'

use master
go
create login testuser with password = 'mypassword123'
go

use test
go

create role reporting
grant select on something to reporting -- grant your permissions here

create user testuser for login testuser
exec sp_addrolemember 'reporting', 'testuser'
go

use test2
go

create role reporting
grant select on something2 to reporting -- grant your permissions here

create user testuser for login testuser
exec sp_addrolemember 'reporting', 'testuser'
go

select * from something
 select * from test2.dbo.something2

declare @sql nvarchar(max), @username nvarchar(50), @password nvarchar(50)

-- ########## SET PARAMETERS HERE
SET @username = N'testguy'
SET @password = N'test123'
-- ########## END SET PARAMETERS

set @sql = N'USE master; CREATE LOGIN [' + @username + N'] WITH PASSWORD = N''' + @password + N'''; USE database1; CREATE USER [' + @username + N'] FOR LOGIN [' + @username + N']; EXEC sp_addrolemember ''reporting'', ''' + @username + N''';  USE database2; CREATE USER [' + @username + N'] FOR LOGIN [' + @username + N']; EXEC sp_addrolemember ''reporting'', ''' + @username + N''';'
exec sp_executesql @sql

declare @login nvarchar(50), @user1 nvarchar(50), @user2 nvarchar(50), @sql nvarchar(max), @rolename nvarchar(50)

SET @rolename = 'reporting'

declare c cursor for
select sp.name as login, dp1.name as user1, dp2.name as user2 from sys.server_principals as sp
    left outer join database1.sys.database_principals as dp1 on sp.sid = dp1.sid
    left outer join database2.sys.database_principals as dp2 on sp.sid = dp2.sid
where sp.type = 'S'
    and sp.is_disabled = 0

open c

fetch next from c into @login, @user1, @user2

while @@FETCH_STATUS = 0 begin

    -- create user in db1
    if (@user1 is null) begin
        SET @sql = N'USE database1; CREATE USER [' + @login + N'] FOR LOGIN [' + @login + N'];'
        EXEC sp_executesql @sql
    end

    -- ensure user is member of role in db1
    SET @sql = N'USE database1; EXEC sp_addrolemember '''+ @rolename + ''', ''' + @login + N''';'
    EXEC sp_executesql @sql

     -- create user in db2
    if (@user2 is null) begin
        SET @sql = N'USE database2; CREATE USER [' + @login + N'] FOR LOGIN [' + @login + N'];'
        EXEC sp_executesql @sql
    end

    -- ensure user is member of role in db2
    SET @sql = N'USE database2; EXEC sp_addrolemember '''+ @rolename + ''', ''' + @login + N''';'
    EXEC sp_executesql @sql

    fetch next from c into @login, @user1, @user2
end


close c
deallocate c