You should use the following combo when you have HS + HU. Then you have:
1. Pop PoM
2. Alter Time
3. First instant pyro from HS
(4. eventually second instant pyro from HU + previous Pyro critting)
5. instant pyro from PoM or when the previous two pyros critted from HU
6. cancel alter time
7. First instant pyro from HS
(8. eventually second instant pyro from HU + previous Pyro critting)
9. instant pyro from PoM or when the previous two pyros critted from HU
10. Do this as long as you get back to back instant pyros
11. combust
12. watch numbers
Even though the instant Pyro from PoM does not benefit from the bonus it gets from HS I think it's still worth to use it with alter time since you will get the effect twice and not only once