And if in last option i use an mux to mux the 10 11 12 13 to the two devices?
Then you wind up with a problem.
However, there is
an "else" available
to actively drive the SS line on both devices all the time. So, no muxing there.
And you can
drive both SCLK and MOSI, so no mux needed there, either.
I checked the Ethernet schematic, and there's a 2-pin jumper that allows SEN to be connected to ground. It should be possible to connect that to a different digital pin, and use that to drive SEN.
Since SEN is active-high, and SS is active-low, it might
be possible to connect the RFM12's SS to the Ethernet's SEN. Well, of course
, but I'm not sure it will work
. ;D There could be a timing problem: since SEN is doing some bus reconfiguration, there might be a time lag before it takes effect that could still cause interference. Or there might not.
Either way, it's worth a try. If it doesn't work out, just assign a different pin to the RFM12's SS, and add delay(s) around the toggling of SEN as needed to make it work.
It's definitely easier than trying to build a mux.