This is DEAD simple actually and it allows you to start to use the Orange Network when you have no T-Mobile signal. (which is better than it trying to go on Orange just because the signal is better).
If you want to opt in to this, you have 2 ways to go about it.
1st, go to the T-Mobile web page right here and follow the instructions.
Your 2nd option is to skip all that and just send a message to 2121 with the body of the message simply saying 'yes'. You will then get a text back confirming your phone has access to the Orange network.
A word of precaution though, this dual network access is really aimed at allowing you to send SMS and make calls. When you sign up to access the Orange service, you are in actual fact only accessing the 2G network from Orange. This will kick up a few data issues, so look at the 'quirks' list below that T-Mobile present before you sign up, to get a bit of a better understanding.
You can always opt out too. Simpy send a text to 2121 saying 'OUT'