This is quite old, but for whoever has the same issue.
Use @PropertyMerge from acs-commons.
You can have separate properties for your tags and have all of them merged into cq:tags property.
<prop1
jcr:primaryType="nt:unstructured"
sling:resourceType="cq/gui/components/coral/common/form/tagfield"
...
name="./prop1"
/>
<prop2
jcr:primaryType="nt:unstructured"
sling:resourceType="cq/gui/components/coral/common/form/tagfield"
...
name="./prop2"
/>
<tagsMerge
jcr:primaryType="nt:unstructured"
sling:resourceType="granite/ui/components/coral/foundation/form/hidden"
name=":prop1@PropertyMerge"
value="cq:tags"
/>
<tagsMerge2
jcr:primaryType="nt:unstructured"
sling:resourceType="granite/ui/components/coral/foundation/form/hidden"
name=":prop2@PropertyMerge"
value="cq:tags"
/>
<cqTags
jcr:primaryType="nt:unstructured"
sling:resourceType="granite/ui/components/coral/foundation/form/hidden"
name="./cq:tags"
/>