Is it possible to have two P7888-2 cards working as if it were a four-channel one?

Yes, you can plug up to four P7888 cards into a PC. They can run in parallel and are operated with the same software. Just edit the P7888.INI file and change the line "devices=1" for example into "devices=2". The P7888 has a resolution of 1 ns when 2 inputs are used and 2ns when 4 inputs are used. So using two P7888-2 has the advantage that a resolution of 1 ns is available in all four channels. A problem could be if you want to correlate events frome inputs between different cards. You have to make sure that both cards are started at the same time and you never loose any start event due to a full FIFO. A much better and more economic solution than using two P7888-2 is the MCS6. It has on each input a 100 ps resolution and you can correlate events from all inputs.