What do you think:

Option 1: First Half vs. Second Half

This is the simplest method. You just split the 16 characters down the middle.

Heads: 0, 1, 2, 3, 4, 5, 6, 7 (the first 8 characters)

Tails: 8, 9, a, b, c, d, e, f (the last 8 characters)

Option 2: Evens vs. Odds

Another very common way to create a 50/50 split is to use parity. For the letters, you can use their number equivalents (a=10, b=11, etc.).

Heads (Even): 0, 2, 4, 6, 8, a, c, e

Tails (Odd): 1, 3, 5, 7, 9, b, d, f

Option 3: Balance Your Original Rule

If you want to keep the "letters are heads, numbers are tails" idea, you can just move two of the numbers over to the heads category to balance it.

Heads: a, b, c, d, e, f and 8, 9

Tails: 0, 1, 2, 3, 4, 5, 6, 7