Max:
Again, many of us only apply a trama damper to damage suffered. Not raw damage taken. RAW is grey on this point and it can be argued either way.
IE: you get hit for 6 points of damage... you roll your soak down to 1. You take one... the trauma damper doesn't kick in since the damage you're actually taking is less than 2 when you're actually marking damage boxes off. There's also some who only apply a trauma damper to physical... others who do both physical and stun. The wording isn't clear on when or exactly what types of damage it works on. (there's a fluff sentence... followed by a crunch sentence which describes exactly how it works for rules and the crunch sentence says (all) damage... while the fluff leans towards physical only).
I've found the more experts types games I've played in tend to do it after soaked damage... and apply it when damage is TAKEN (checking off boxes on your sheet). The more pink mohawk games do it before rolling the dice for a far more run and gun game. Since the rules never say which is correct, both are reasonable readings and dependent on the table.
Just so you know.. here's an easy formula i worked up to figure out odds in a hurry. COMBIN(NumDice, NumSuccess)*(2^(NumDice-NumSuccess)). Will tell you exactly how many cases out of 3^(NumDice) possibilities result in exactly that number of successes. You can double check that... the riemann sum from 0 to NumDice of that formula exactly equals 3^(numdice) for any number of dice you choose so it always produces 100% of the possible results broken down by successes.
So using your 10 dice drain... and 2 drain result (don't forget to increase drain by +1 per spell after the first for multicasting).
With 2 drain. You have 1024 cases of 2 drain (no successes), You have 5120 cases cases of 1 drain (1 success)... and we don't care about 2 or more as all of them result in no drain. Out of 59049 possible results. So you have ~0.2 drain per casting... with a good size drain pool. Since you're casting it twice each time. The average is times 2... goes up to ~0.4 drain per casting... or a little under 50% odds each time you cast that you take a point of drain! (hardly insignificant).
If you overcast at force 11 right away... 4 drain. 1024 cases of 4 drain, 5120 cases of 3 drain, (note how the existing cases all had their drain go up significantly), 11520 cases of 2 drain, and 15360 cases of 1 drain. Out of 59049 possibilities. Average: ~0.98 drain per casting. So each time you do that you probably take 1 drain.
If you don't engage the optional rule and you assume force 7 with say 3 net hits for free damage... the average drain per casting is again drain 2... so again ~0.2 drain per casting. But instead of needing two castings (with less dice per casting... increasing the odds of resisting or glitching). You only need one... so the drain is less than half what multicasting is on average. (if you account for higher failure rate on lower force, lower dice pool split castings).