In September, we reported that Orange and T-Mobile were offering roaming across their networks, so T-Mobile users would be able to call and text on the Orange network, and Orange users could do the same on the T-Mobile network.
T-Mobile explains that when you roam onto an Orange network, you will get access to the 2G network - mostly used for voice and texting, although it adds that you may some "basic, slow internet and data services."
Orange says in its FAQ that Orange users roaming onto T-Mobile will only get calls and texts, but that it is planning to include 3G roaming in 2011.
Sharing the Orange / T-Mobile signal is easy but you need to enable it.
The links to Orange and T-Mobile below will get you set up:
For T-Mobile customers: Enable Orange roaming
For Orange customers: Enable T-Mobile roaming