I keep my nails short and I have a hard time with cans, too. Though not a hard issue to solve, even a key should be enough to bend the tab enough to get purchase with your fingers.

The easiest way (for me) to separate legos stuck like that is attaching a standard 2x4 block to each, top and bottom, lay it down on its side, then pick it up and use the 2x4s for leverage to pry it half down the middle.

You can also put a 45-degree v-slope block under the seam and push down on either side for more leverage.