A very good quality Y-split from Cordial. This cord combines two mono signals into one mono jack, or alternatively you can split a mono signal two ways. The unfortunate thing about Y-cables is that the cheaper ones are often plagued by contact failures. Luckily Cordial can be trusted and I have had no problems with these. I use a total of four PGGs to split the signals from the left and right channels of my two synthesizers, i.e. from one channel one mono is going to the mixer and the other mono is connected to the audio interface.
And extra kudos for the gold plated connectors, as they protect the jacks from oxidation (rather than affecting the sonic quality). The REAN connectors are another good thing, as they are easy to open and fix if broken.