type SomeStruct struct {
     Campaigns CampaignsNode `xml:"campaigns"`
}

type CampaignsNode struct {
    First CampaignsGrouping `xml:"first"`
    Second CampaignsGrouping `xml:"second"`
}

type CampaignsGrouping []Campaign

type Campaign struct {
    XMLName   xml.Name `xml:"my_campaign_tag"`
    Id int
    Name string
}

<SomeStruct>
    <Campaigns>
        <First>
            <my_campaign_tag>...</my_campaign_tag>
            <my_campaign_tag>...</my_campaign_tag>
        </First>
        <Second>
            <my_campaign_tag>...</my_campaign_tag>
            <my_campaign_tag>...</my_campaign_tag>
        </Second>
    </Campaigns>
    </SomeStruct>

<SomeStruct>
    <Campaigns>
        <my_campaign_tag>...</my_campaign_tag>
        <my_campaign_tag>...</my_campaign_tag>
        <my_campaign_tag>...</my_campaign_tag>
        <my_campaign_tag>...</my_campaign_tag>
    </Campaigns>
</SomeStruct>