Having the same physical memory layout on both channels is the only requirement to dual-channel. Whether or not the memory configuration will be stable or even work at all due to variations between DIMMs, motherboards and CPUs is a different story, especially at higher frequencies.
both brands must be same clock speeds and timigs so you keep the dual channel feature available
The only requirement for dual-channel is that memory loading has the same layout on both channels. The timings don't matter, you just end up running all the memory at the slowest frequency and timings of the set to accommodate the worst timings of the lot.
yes and no, the more different the modules are, the more likely the mainboard wil decide that it will not be dual channel even if most requirements are met
Having the same physical memory layout on both channels is the only requirement to dual-channel. Whether or not the memory configuration will be stable or even work at all due to variations between DIMMs, motherboards and CPUs is a different story, especially at higher frequencies.