Cant be sure of the plec in your profile. But yes you can have more than one in a tank depending on the tank size and how big they will grow. I.e. dont have two commons in a 24" tank (or even one )
I don't see your profile for some reason but I can tell you that unless it's a huge tank and a small species of pleco, they will fight for territory so it's generaly best to keep just one. There are exceptions but I don't know what you have so I can't be any more specific than that.
I have two common plecs together but they are less than a year old and they get along well for now (they are always hanging together). Though when and if the time comes one will be moved to a different tank.
Just the other day I saw a couple of common plecos at my LFS in a goldfish tank and they were fighting for all they were worth. They were both about 2 inches long. I think everything depends on the situation and their environment but I still wouldn't reccomend keeping 2 in the same tank- plecos get big!
Well the pleco in your profile looks like a common pleco meaning it will get quite large. If you would like to keep plecos together most of the time they needed to me young when put together or exactly the same size and you must have a large enough tank with plenty of hiding spaces for they are terratorial. I Have a chocolate pleco (In siggy) a common pleco, and a candy striped pleco all in the same tank happily!
i think if they all have their own little section of the tank which they have claimed as their territory, plecos should be fine together. i had two small bristlenoses in the same tank once and they never faught
ecs can be kept together no problem for the most part, what it comes down to is the same as any fish and tank. A matter of space, territory, volume, filters and other usual things.
If what you have is indeed a common plec, don't get another. That'll just amount to 2 12" fish that you'll need to re-home or a tank twice as large to upgrade to.