I know you're much more dedicated to the number crunching than I am, so if you ever find a way to calculate the DPS difference between the capstone vs the second SLA MM in combat situations, I'd be interested in what you find.
The TL;DR answer is, it's not close - the capstone wins by miles. Splashing 1 Wizard is only worthwhile if you're not planning on leveling past 20.
Here's the long answer.
There are a lot of variables to account for depending on the exact build, varying cooldowns on the spells in the rotations and what spellpower, crit chance and crit damage multis you're looking at, so it is difficult/impossible to give a specific number, but in this case that's not needed.
In it's most simplistic form, you're basically asking if an additional MM SLA is more net damage in an ATr's SLA/Spell rotation than the next best SLA/spell you'd use if you don't have that second MM SLA.
So the question at it's core is, how many additional Ambush procs would the second MM SLA add to the rotation versus the next best option AND are those additional procs enough to make up for losing 50% damage on every proc in the rotation?
Let's assume a 5 spell/sla rotation.
The no-capstone rotation would be something like somewhere around 22 procs - MM SLA (5), MM SLA(5), Force Missle SLA(4), Chain Missle SLA(3), MM spell(5).
The capstone rotation would be something like 20-21 procs depending on what SLA/spell you used insead of the second MM SLA: MM SLA (5), Force Missle SLA(4), Chain Missle SLA(3), MM spell(5), Force Missle spell(4).
1-2 additional Ambush procs wouldn't come remotely close to making up for losing 50% of the damage on the other 20-21 procs in the total rotation.