Apple mail defaults to wrong addressee on Mac

I can think of three different ways of composing an email to avoid confusing these two contacts of yours.

1. You could type in their second name. This will also provide a list of results of names matching the text you have entered. Presumably these two individuals have different surnames and won't then appear together.
2. Start your email to them from the Contacts app. This is a bit more of a roundabout way of starting an email but it is a manual check on your part that you have selected the appropriate person. Find their Contact entry, hover the pointer of the email you wish to receive your message and then click on the small blue mail icon. This will generate a new email to that address.
3. You could use Automator to create a workflow and call it something like 'Email to A'. This then can be accessed in any application using the dropdown option Services under the name of the active application. Again, this is a manual way of creating an email but if as you said you want to be clear about getting the address right it might be a good option.

*See Automator User Guide for Mac. This application looks daunting, and it can be, but for simple tasks such as the one I've described it is relatively easy to set up.