Since you're specifically asking for a plugin and not a technique, I can suggest a Roland Space Echo (Soundtoys Echoboy, GSI or UAD). It's great for adding
a little depth to brass. I assume that's one of the things you wanted to achieve by layering it with a detuned version. Some plugins that also work great
on brass are 1176 compressors and SSL EQs/Channelstrips. But it's very hard to give a valuable advice by looking at your description.

First, you need to find out where the problem is with your brass, and then do a little research yourself. I don't wanna sound like an internet forum
smart*ss, but it's better to know the technique then the plugin that might help you to achieve the results you are looking for.

Big Mike