Everything I have seen on the ground points to Three giving up their existing base stations and moving their equipment into the nearest T-mobile site or should I say sharing T-mobiles existing 3G equipment.
But to remain seperate, they won't be merging, they are using seperate antennas and this is where the problem lies.
Both 2g and 3g networks use recieve diversity to improve coverage and reduce noise, to do this, each cell uses two antennas, one to transmit and recieve and one to recieve only, the two recieve signal are the compared, the result is a better recieve signal, ie. from your phone.
Now what they have done, rather than fit new antennas for Three to use, they have dumped recieve diversity and are using the second recieve antenna for Three.
Noise is a bigger problem in 3G networks, especially phase noise. This cheap fix will have a detrimental effect on coverage and will lead to more dropped calls.