I'll show lcm $(a,b)$ by $[a,b]$ and $\gcd(a,b)$ by $(a,b) $.
$$[m,n]+[m+1,n+1]>\frac{2mn}{\sqrt{n-m}} $$
This is my attempt:
\begin{align*} m-n&\ge (m,n)\\ \implies [m,n]&\ge\frac{mn}{m-n} \end{align*}
so we have :
\begin{align*} [m,n]+[m+1,n+1]&\ge\frac{mn+(m+1)(n+1)}{m-n}\\ &=\frac{2mn}{m-n}+\frac{m+n+1}{m-n} \end{align*}
and here I don't know how to continue.