Symlinks are the simplest solution (just checked, it works)

$ tree test-role*
test-role1
├── tasks
│  └── main.yml
└── templates
    └── template1.j2
test-role2
├── tasks
│ └── main.yml
└── templates
    └── template1.j2 -> ../../test-role1/templates/template1.j2

$  cat test-role1/tasks/main.yml 
- name: copy template
  tags: test-role1
  become: yes
  template:
    src: template1.j2
    dest: /root/template1

$ cat test-role2/tasks/main.yml 
- name: copy template
  tags: test-role2
  become: yes
  template:
    src: template1.j2
    dest: /root/template1

$ ansible-playbook playbook.yml --diff --check --tags test-role2
...
TASK [test-role2 : copy template] *****************
***
before
+++ after: /home/deploy/.ansible/tmp/ansible-local-143602q8imzfz/tmp8g6dn_h1/template1.j2
@@ -0,0 +1 @@
+this is a template