What you have currently will cast only Spear Hand Strike when shift is held, else cast Tiger Palm.
If you want them to cast together, you could do this:
/cast [mod:shift] Spear Hand Strike
/cast Tiger Palm
That will cast Spear Hand Strike and Tiger Palm when shift is held, else just Tiger Palm. Note that this macro only works because interrupts are not on the GCD. Additionally, if you are on GCD, this macro will only cast Spear Hand Strike, which is good. They took interrupts off the GCD for a reason.
I would also consider not macroing your interrupt in with anything else. Interrupts need to go out very quickly in most cases, and fumbling with modifiers may slow down your reaction time. But if it works for you, it works.