Definitely do that - there are very few reasons the average user would need 2 GPU's, so save your money and use the more powerful one (the 570).
I have an RX 480 8Gb card. My main monitor is a 1440p 75Hz Freesync monitor. My secondary is a plain vanilla 1080p 60Hz monitor non-freesync. The 480 has no trouble running both. The only thing is when I have both monitors enabled, idle power consumption goes from about 8 watts to about 19, according to MSI Afterburner. Go ahead & skip the second card. Also probably wouldn't be a bad idea to look at the used market. 8Gb 480's & 570's can be found for around $100 to $120 right now. They're an absolute steal at those prices. Performance is pretty close to identical between them.