I read in the documentation that the TLP model can count pulses from 2 devices like Water or Gas meters. But it looks like the voltage of the source of pulses must be 230V. Is this right ? If so, how could a meter with a reed relay or an open collector output be connected to the LTP meter ?

Model LTP, LT has 2 digital inputs 230V and can “read” 230V AC pulses at frequency 1 Hz or 200V DC at frequency 50 Hz, so you need an adapter to be able to connect water or any other pulse meter to LTP.
You must enable option DIN Energy option under Technical menu – Option in order to use this function. Energy is then shown Under Energy View – D.Input Energy, where you can set k1 and k2 factor (pulse weight).