I had a molly miller and a bicolor blenny in the same 55g years ago and they got along. Most of the time fish that are this similar will fight. IMO mine got along because they filled different niches. The molly miller is an omnivore but this particular one preferred meaty foods and rarely ate algae. The bicolor ate mostly algae and on rare occasion ate meaty foods. Hence, I think that is why they got along. I think that there is always a risk when putting more than one blenny along with another, but it depends on the species and the "personality" of your particular fish. If the water quality is good, food is plentiful, then more times that not things will be OK. But then again, it could go wrong too the minute you move your new blenny from quarantine to your tank. It might be a good idea to rearrange things a bit that day especially you lawnmower's favorite perching spots.
Now, this is really a generalization. I did a google search on "blennies" and saw one pic with three tompot blennies hiding in the same hole. Also, the more peaceful fanged blennies get along with their own kind and often similar other species as well.
Also, in large oyster reef ecosystems in my home waters, there is one species of blenny (the striped blenny, Chasmodes bosquianus, carnivorous species) that basically colonizes the area. Males and females differ quite a bit in appearance and you may see more than one blenny hiding in an oyster shell. That doesn't mean that there are threatening encounters by rivals over a territory, but in a larger system (duplicated in aquaria) the encounters often don't result in injury or stress...they eventually go about their own way. I posted a link to a movie in another blenny thread.
Good luck. If it works out for you then you'll have twice the fun!
