Principā latviskais jau ir drusku neprecīzs. Plus, kāpēc divi teikumi, kas pasak būtībā vienu un to pašu?

Bet anyway:

For the second leg to be at the same angle as the first, it must not be screwed in fully, thus making it (vai construction) unstable.

If the second leg is screwed in fully, it is shorter than the first one and the chair is then unstable.