Every faction can only have one Titan built at any given time. Having multiple titans would be insanely overpowered so that's why it is limited to one for each player.

At the moment it is possible to build multiple titans at once though (up to a maximum of 8 titans in total). If you build multiple titan factories (one factory per gravity well) and start building all titans at the same time, you can have more than one titan in the game.

This is correct.  Ironically, the bug is the fact that on some comps (not on mine), you can actually queue more than 1 Titan at a time.  It's when you've built the foundry, and haven't researched the 4th Titan research yet.

I know this because I tried to take advantage of the bug and build more than one Titan.  It didn't work.

But, yes, the one Titan limit is part of the game.  We can't have too many gargantuan machines of flying death around, can't we?

<evil laugh>