I thought your problem is:
but mining Cryptonight
1 of then ash at 750h\s and the other one 450\h\s
Focus on your main problem first, seems you don't have an issue on Ethash, those two cards, do mining with the same Hashrate on Ethash. Did you try my suggestion above?
oh no on ethash bot cards do the same
but i didnt udnerstand what do you mean dummy output !!
If you've done what I mentioned before and didn't fix the problem, make sure you're not connecting your monitor to any of your rx570s. Connect your monitor directly to the motherboard integrated graphics.