Is it possible to create context from multiple platforms?

I’ve fetched platform id’s and was wondering if it’s possible to create one context from all of them? I’ve tried to feed the id’s to cl_context_properties array and ending it with zero with no luck. Somehow I’ve got impression this should be possible.

Question number two:
queue = clCreateCommandQueue(context, NULL, 0, NULL);
in some examples second NULL is replaced with device id. Does this one with NULL instead device id send work to all devices in context or what?

I’ve just started tinkering with OpenCL and sticking to pure C, so if you got some samples to look at, I’d appreciate if they were plain C.

A context is linked to only one platform, and command queue must be linked to one (and only one) device.