I don't believe there is any possibility that the Leafs pursue both ROR and Tarasenko. There likely isn't a way to make the cap work, and the Leafs really don't have the assets.
Of course, ROR would be the preferred target. If available in the rental market, he's probably the #1 commodity. I'd imagine that you're talking about at least 2 of Knies/1st/Sandin or Liljegren plus another asset.... maybe even all 3 of those pieces.
There's also the challenge in that the only way to fit ROR in is with Muzzin forsure done (or you move Kerfoot and do some minor finnagling elsewhere to make up the $250k). If Muzzin is done, a blueline that consists of Rielly, Brodie, Holl, Sandin, Liljegren & Giordano is the obvious weakness... even if they have another 4 or 5 NHL-quality dmen that can step in for injuries.
If Dubas was a GM that's 1-2 years into the job, then I'd say there's no way he blows that kind of haul on a rental like ROR.
But, given that Dubas seems to be on his last leash, and 2 of your biggest stars will each be a year away from UFA this summer, it seems like the exact "all-in" move that Dubas would probably be best served making.