You put the linux sd in raspeberry PI and you boot.
Open config drive and run the config tool change the acsi ids to be 0 for the sd and 1 for the cosmosex and save.
On the next boot you will boot to ppera's sd. (p1)
In order to read both cosmosex(usb/network) and the ppera SD you need to put the cosmosex drivers in the auto folder of the ppera's sd according of the guides from jookie here : http://joo.kie.sk/?page_id=415
Some tips : the raspberry PI (the 3 I suppose) needs an adeguate power unit to work good.
It is essential also for microscosmos the power to be enough to keep the connection good.
Use a floppy to copy from the system booting only to cosmosex to the system booting to ppera's sd the drivers in order to have both active