I'd like to make a Y cable from my Winkeyer to the 5K and the other side to the 6K, any problem with

Bill,

Since we don't have access to schematics for either Flex transceiver, it's probably safe to assume the key line for each uses a pull-up resistor to +5V or a similar low voltage.  If so, then at a minimum, you should use a steering diode on the cables connecting to each transceiver to attain reasonable isolation.   Otherwise, you're placing two differing source voltages in parallel and that's poor engineering practice.   I now fully isolate my key lines with MOSFET devices, but at one time, I would just install a 1N914/1N4148 diode inside the male RCA connectors and label them as such. 

The WinkeyerUSB has two KEY OUT lines, so you may be able to simply direct the keying to one of the two key ports.  But, I'm not sure if that's the Winkeyer device you're using -- or maybe one of the older, original kits?

Paul, W9AC