How to fairly rank the best items having two set of grades?
Procedure-1:

Convert them into percentage and then take average of outcome of both.

so for example for person A, judge-1 rated 5 out of 7 and judge-2 rated 7 out of 10,

converting judge-1 score to percentage i.e., $5/7 *100$ = 71%(rounded 71.45)

converting judge-2 score to percentage i.e., $7/10 *100$ = 70% 

Now take average of both i.e., $71+70 / 2$ =  70.5%, now you can convert it back on to a scale from $1-10$ :  This person A's overall rating is 7(rounded 7.05 ) out of 10.

Procedure-2:

You can normalize the rating of both the judges by using this formula, 

$$z_i=\frac{x_i-\min(x)}{\max(x)-\min(x)}$$

where $x=(x_1,...,x_n)$ and $z_i$ is now your $i^{th}$ normalized data.  As a proof of concept (although you did not ask for it)

Procedure-3:

You can convert the 0-7 rating to 0-10 and then take average of both.

Once you get the outcome then you can rank them accordingly.

Do let me know if you have any doubts.