I found out that Huawei P8 (not lite, not plus) supports dual sim, when I can insert only 1 sim and 1 sd, without possibility to swich sd for second sim. I tried micro and nano sim and none fits to sd slot. Are there two versions of Huawei P8? Is it possible to check whether my phone supports dual sim?
There are several variants for P8:
GRA-UL00 - LTE/WCDMA for China Unicom ("open"), dual SIM;
GRA-TL00 - TD-LTE/TD-SCDMA for China Mobile, dual SIM;
GRA-CL00 - LTE/CDMA for China Telecom, dual SIM;
(the 3 above comes with GRA-*L10 sub-variants with 64GB storage instead of 16GB)
...and finally GRA-L09 - international LTE with European bands, single SIM.
The dual SIM variants have one slot for either a nano-SIM or a MicroSD card, and a dedicated slot for a nano-SIM, as shown below. You did mention that one SD and one nano-SIM can be inserted at the same time - that means you do own a dual SIM variant, just didn't get the correct way to utilize it.
Huawei P8 has dual SIM support. You should put the 2nd nano SIM "vertically" so that it fits into the slot.