2 way 1 / 2 gang isolation

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We currently have 1 gang 2 way to a single light. We are installing 5 downlighters instead, and I want them to switch in a set of 3 and a set of 2. We should therefore change to 2 gang 2 way circuit. However... We can't trace the cabling between the switches and the lighting point (lath & plaster walls). This is a temporary fix until we gut and extend the house in a couple of year's time. We know that the switches work the light. Full stop.

What we have done is install a junction box from the lighting point backwards if you like, and split it into 2 routes - 1 to the 3 light group, 1 to the 2 group, isolating this with a switch. Not working though. All 5 lights work, but it trips when the isolating switch is flicked.
Anyone tell me why and how to sort it?
Many thanks...