I agree with the above advice; different genera with differing body shapes has a better chance.
But I would add that the habits of the fish play the most important role. If both blennies are perchers that pick algae off of the rocks, you could have trouble; even if different genera. I was advised against adding an Ecsenius sp. blenny with a Hypsoblennius sp. blenny unless my tank was at least six feet long. The fish do set up territories and defend them aggressively. A 55gal may not be big enough for more than one blenny.
And with all fishes, each fish has its own personality. I had a Starry Blenny that was an absolute terror even though I know of plenty others that do just fine in a community reef.
