I managed to do the Dual boot on one drive. It seems to be correct. I can boot both systems without USB. Maybe I can make a guide later. (I erased the disk and then made 2 partitions HFS+, installed El capitan on 1.st partition, then installed Windows on the second partition by deleting the 2.nd partition and reformating to NTFS, booted from USB, use multibeast, correct EFI folder on SSD and add a BIOS entry to boot from this EFI partition)
NOTE: You need to have only one disk inserted in laptop so that partitioning will create EFI folder on this one and not the second one.
I'm having troubles patching DSDT.dsl though. Can't get rid of some syntax errors. Do you think that this happens due to bad DSDT.dsl extraction or I have to fix it later on with MaciASL? This error makes no sense to me.