The (GWR-sponsored) Ffestiniog and Blaenau Railway connected with the Ffestiniog Railway, and was originally built to the same gauge. The GWR ensured all bridges were built to allow widening to standard gauge, and unsurprisingly, this was done quite soon by the GWR after taking over.
On the (pre-) Welsh Highland side of things, both the Croesor and Gorseddau Tramways were operated at 2ft gauge, the Gorseddau being relaid to match the Croesor, which it met at Porthmadog Wharf. But given that these pre-date the WHR, and were partially absorbed by it, I'm not sure if they match what you had in mind.
Edit: I misread the question as 2ft-gauge lines that were connected, as opposed to ones that were connected to the FR/WHR.