How does the game decide who goes first when two toons are BOTH granted 100% turn meter?

It's not a coin flip according to devs.

Each character has its own speed pacing and internally it's represented with large numbers, not a 1-100 range integer like the percent representation leads you to believe.

So even though two chars are on 100%, the chances of both being the exact same value internally is (according to devs) close to zero. If a character naturally gains more than enough to get a turn, the excess bleeds into his next turn. But if he gets a TM buff, the excess beyond 100% is lost.