As Ban quite rightly says, it depends upon how your switches are wired and to a certain extent how the roses are wired as to what route you take to alter the wiring.
If one rose is wired conventionally (Feed in Feed out and drop to switch) with another switched feed linking to the other rose,and if one switch is wired as a one way that has a three core cable linking to the second switch then what you need to do is install an extra wire to link the second rose to the second switch.
It is quite straight-forward, provided everything is as described above:
Install a link as suggested between the rose with all the wires terminating in it and the switch with the two cables.
This wire you then connect (at the rose end) to the live conductor which feeds the second rose, using a terminal block or crimp connectors. The Neutral & Earth can be left as is.
At the switch end, you disconnect all the wires from both switches and reconnect as follows:
In the switch with two cables:
The twin and earth switch cable connects to Common (live feed) and L1 (return or switched feed).
This will now switch the ceiling rose with all the cables in.
In the same switch, the new switch link you have installed connects to the yellow conductor in the three core, while the red connects to the common terminal of the switch. The blue can be disregarded. Don't forget to mark the yellow as a live conductor using red tape or sleeving.
Tape up the blue so it cannot touch anything else.
At the switch with the three core cable, use the red as Common and the yellow into L1. Tape up the blue again.
This switch will now operate the rose with one cable supplying it.
There are two problems I have thought of while typing this up; one is that you may want the switches the other way around, and the second thing is you may only have plaster depth boxes, in which case putting extra connections in there may be a bit tight, but you could drill out the wall and fit a 25mm depth box.
