Quite honestly, the variant covers for Batman kinda suck. The "Night of the Monster Men" covers were particularly bad (both regular and variant).
They are paying two artists with the same price they were paying one before, is my guess. The cover of Flash Rebirth 1B is horrible.
Interestingly, it doesn't seem to have bumped sales. DC has two covers for each issue. Batman is really only doing the same numbers it was during New 52 +/- 10,000.
It is hard to say what impact it has had on sales. Batman written by Mikel Janin and/or Tom King (who are they?) is selling as well as Batman written by Scott Snyder. Pretty impressive when you consider how popular Snyder's run was. Do two covers for regular price help? It is hard to say, but it definitely isn't hurting.
Scott Snyder had a great run on Batman, one of the best. He has some great comics under his belt, but I think that series propelled him. Tom King is somewhat well known. Yes, he was not as well known in comic circles.