Simultaneous damage might be very interesting if it was properly applied to the following modifiers as well (beyond first strike and first strike counters). (I'm using counter-attack below as a description of the attack the defender performs in a simultaneous combat situation)
UnitStat_CounterattackMultiplier - currently a unitstat that modifies the counterattack damage from the unit. Ie, the counter-attack is different from the standard attack by a variable. Could be stronger, could be weaker. Pikemen may be very strong at counter-attacking, but not so strong at attacking.
<Attribute>CannotBeCounterattacked</Attribute> - Unit can attack without suffering simultaneous damage. This may be limited to 1 per turn or something like that (ie - the first attack this unit performs in each turn cannot be counter-attacked). That could also be limited to "the first attack this unit performs each combat cannot be counter-attacked".
<Attribute>DoesNotCounterAttack</Attribute> - Unit never counter-attacks. This should be available both as a flaw-trait on units, and a possible debuff on enemy units. Similarly, it may be limited to "the next attack suffered" or "the next 2 attacks suffered" and so on. It could also be "does not counter-attack for the first attack suffered each turn".
Edit: A new attribute that might be fun, especially for shield-dependant units or "boss type" monsters. This attribute would allow the unit to ignore the first attack it suffers in a turn or combat, and still deal counter-attack damage to whoever attacked.