What price are you seeing for the 1070s? Are you buying new? The prices of 1070s tend to not be particularly attractive at this point, since they only perform about on par with a 1660 Ti, which start at around US $250 new. The 1660 Ti does not support SLI, but for what it would cost for a pair of 1070s (at least in the US market), you could likely get something like a single 2070 SUPER, which performs roughly on par with a 1080 Ti, and offers some acceleration of raytraced lighting effects in the few games that currently support them, for not much more than $500.
SLI tends to have questionable support these days, with a number of games not working properly with it, and in those cases you might be down to just one 1070. And of course, there's the higher heat output, power consumption and noise from running two cards. It's possible that there may be some examples of games where the SLI setup manages to push higher frame rates, but overall the single faster card would likely provide a more consistent experience.
As for whether that particular board supports multi-card setups, looking at its product page, I'm only seeing crossfire support (for AMD cards) listed, and while some of MSI's X570 boards apparently support SLI, it doesn't look like that one does. Again, you are probably not missing much unless you can get a pair of 1070s for significantly less than the cost of something like a single 2070 SUPER though.