could be a few different ways of doing this, could you give a clearer description, can't totally understand your post.
it may not been wired correctly in first place.
do your strapper wires (yellow,blue,red) go between switches and then switch wires(red,black) to light
As Kbrownie suggests, it is possible that they weren't wired correctly in the first place. However, we have to start somewhere. In each of your 2g2w switch plates, you could expect to see a red and black "twin & earth" cable, and the red, yellow and blue "triple & earth" cable. The twin & earth cables in each switch plate will be supplying the hall and bathroom lights respectively with red to COM and black to L1 of one switch in each switch plate. The red cores are permanently live, and black cores only live with the switch closed. The second switch on each switch plate will be wired to the triple & earth cable for the two way operation of the landing light. Personally, I would follow convention and assume that red cores are those that are normally "permanently live" and thus, these should be connected to the COM terminals. The yellow and blue cores, connected to L1 and L2, will essentially link the two (downstairs and upstairs) switches together. There aren't enough terminals (or space!) in a ceiling rose for a two-way wire up unless supplemented by connector block, which is messy, so most likely, there will be a junction (looping-in) box somewhere inbetween, where the wires going to both switch plates are connected up. The red core (of the triple & earth) going to ONE of the two switch plates, will be connected to the terminal in the junction box to which is connected, the live feed. (referred to as the "permanent live"). The red core of the triple & earth cable going to the other switch plate will only be live when supplied via either the yellow or blue cores depending on switch positioning. This red core (the "switched live"), will be connected to a "separate" terminal in the junction box, along with the brown core from the light fitting flex. The yellow and blue cores will either pass through the junction box unbroken, or be joined (y to y, bl to bl) at their own respective terminals. It is possible, that in the 2g2w switch plate that has the permanent live red for the landing light, a short wire link exists between both COM terminals, which will provide the permanent live to the other switch rather than via the red core of a separate twin & earth cable.
Long-winded I know, and possibly not a match for your situation but hopefully will give you some inspiration!?
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