I dont know how to perform crossover with 2 amd vega 56 cards can any one help me
If you mean crossfire not crossover then this page gives some great details:
How to Configure AMD CrossFire™ Using AMD Radeon™ Settings
i don't have this option in my software
You may need to enable "Crossfire" in the motherboard BIOS.
INFORMATION REQUIRED WHEN POSTING A QUESTION
I don't crossfire myself. Never found it to be worthwhile. Typically created more issues than it helped.
Found this article that may explain things a bit:
AMD Distancing from CrossFire with RX Vega | GamersNexus - Gaming PC Builds & Hardware Benchmarks
Retrieving data ...