Hi Mikko!
I agree to Onki: if you fly with Jeti Duplex, you don't have to use the optional channel.
I also fly with Jeti (DC-16 and DC-24) and have 4 GPS Logger 2 and 1 GPS Logger 1, all in my sailplanes. The transfer from GPS Logger works via EX-telemtry, so you can do anything you want right at the Jeti Transmitter and don't need to connect the optional channel.
Best regards...
Frank