Most weddings I have attended have seen the Maid of Honour and Best Man chosen as legal witnesses. This is certainly a viable (and sometimes easier) option, considering that these two people will be stood next or near to you on the day. However, there are some aspects that you may not know when it comes to determining your legal witnesses:
1. You do not have to know them.
Yes, that's right. You can actually pull two people from the street and ask them to witness your ceremony. Crazy? Maybe. But couples who elope or would like to add some secrecy to their ceremony are able to include this legal aspect seamlessly.
2. Witnesses must be able to understand what happens at the ceremony (They should be of sound mind, and should not be intoxicated).
So, if you plan on including your Maid of Honour and Best Man as legal witnesses, watch what and how much they drink on the day. Of course this "rule" extends towards other aspects such as mental health and disability, so make sure that you account for these instances too. The celebrant needs to be sure that your witnesses understand and agree with what the ceremony represents.
3. Children can be witnesses.
That's right! But they must be old enough to understand the nature of the ceremony, and must be able to demonstrate that understanding in court if a situation should require them to.
As you can see, there is an aspect of variety when it comes to choosing your witnesses. What is important to remember is that it is your choice and as long as you have two people who have agreed to sign your license, are of sound mind, and understand the nature of the ceremony, you have made steps towards ensuring the legality of your union.