Making a wedding cake and the bottom 2 tiers (4 tier cake) is carrot cake with cream cheese icing.

I need the sturdiest possible cream cheese icing.

I know to have the butter at room temp and the cream cheese cold, and that the more you beat it the softer it gets.....what else?  What ratio of butter and sugar  to cream cheese works best for this?

if the cream cheese is cold it will make lumps--so you could sieve it to eliminate those

i'd suggest doing an online search for safe cream cheese icing--i know there have been recipes floating around that have been tested to be sure the sugar content is high enough to preserve the cream cheese icing--and inhibit the bacteria from becoming hazardous--

i'm sure you can locate this with a search if no one posts it for you--

you want to be safe with this and that recipe will also be sturdier--

regarding the sturdy factor--add whatever liquid the recipe calls for super sparingly like probably not at all ;)

make a test batch too 

I add dream whip to my cream cheese frosting to make it more sturdy..also to my cannoli filling. :)