Planning to glue 2 pieces of 9 mm oak planks to make 1 piece of 18 mm. (Wood glue and clamps.) And then make a threshold from this. Will it hold? I know wood glue becomes strong but a threshold is supposed to last long and is subjected to great forces. Strong friends 
Thinking of gluing it.
Intend to ensure it lies flush against the surface so that it stays really tight.
Wooden beam on concrete slab. Unfortunately, a bit of linoleum sticks out from one side and some uneven leveling compound from the other. But perhaps I can remove that to get a more even surface.
No problem. I carve them underneath towards the surrounding floor so there is no gap, and the threshold goes over the floors a little. I also make them with a smooth top. Then I don't know if 9mm is defined as plank
True. 9 mm is more like logsA AndersS said:No problem. I groove them underneath towards the surrounding floor so there's no gap, and the threshold goes over the floors a bit.
I also make them with a smooth top. Then I'm not sure if 9mm is defined as plank