Power supply and hashrate question for R9 390 8GB & R9 280x

I am starting mining with a R9 390 8GB today and i am thinking of adding a R9 280x.
Does anyone have a sense what my hashrate will probably be?

And i am using this PSU https://tweakers.net/pricewatch/254690/seasonic-m12ii-620w.html
Will that be sufficient to power both?

Thanks!