I'll tackle Artyom first. My first reaction was to miss out the 'y' altogether and pronounce it ART-em. But then I thought that didn't make such sense so with the 'y' I thought may be ART-yum. And then my third and final thought, and probably the one I like the most (though it took me the longest to reach) was art-EE-ohm.
Petar is a little easier. I'd either go with the English pronunciation of Peter (PEE-ter) or put the emphasis on the second syllable and go pee-TAR.
Will be spreading some love to Thailand's vulnerable children and animals summer 2016 ~ please support this worthy cause :-)
gofund.me/epv33uzf
http://www.behindthename.com/pnl/117870?sort=rating&pop=ew^Top 2 = Eve and James // Bottom 2 = Indiana (m) Robertina