I'm going to take a guess at what you're after - feel free to ask me to adjust the markup if this isn't what you tried to describe.

> I cannot seem to get the button to cover both columns due to it being constrained within the `slds-size_1-of-2` parent div

The key to this is to get the button out of that parent `div` with the `1-of-2` class on it and into its own `div` which will take up 100% of the parent container (the form) width.

    <aura:component implements="flexipage:availableForAllPageTypes" access="global" >
        
        <lightning:recordEditForm recordId="0036A000001eZcVQAU"
                                  objectApiName="Contact">
            <!-- the messages component is for error messages -->
            <lightning:messages />
            
            <!-- fields in the left column -->
            <div class="slds-size_1-of-2">
                <lightning:inputField fieldName="FirstName" />
                <lightning:inputField fieldName="LastName" />
                <lightning:inputField fieldName="Email" />
            </div>
            
            <!-- button in its own div in the center of the lightning:recordEditForm container -->
            <div class="slds-align_absolute-center slds-m-top_medium">
                <lightning:button variant="brand" type="submit" name="save" label="Save" />
            </div>
        </lightning:recordEditForm>
    
    
    </aura:component>

Which renders like this:

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/SN7Lu.png