Some games -- Hackmaster and Aces and Eights, both from KenzerCo., come to mind -- use a second by second clock that is about as close to "simultaneous" as you can probably get at a table. So, for example, it might take 4 second to swing your sword and 3 seconds to cast a quick spell but 10 seconds to cast a more powerful one, and so on. The GM just counts of seconds and when your wait time comes up you act, say what you are doing next, and keep going like that.
No, there are more simultaneous systems that saw print. And they're more simultaneous than HM/A&8. Tho' I'll note that the playtest for Mongoose Traveller 1E had a similar modality, except that it was count up to max 6, at 6, you can take an action. (at 1-5, you can react)... which is about equally as simultaneous as A&8 or HM.
Ones that are more simultaneous:
Technically,
Mouse Guard,
Burning Wheel,
Burning Empires, and WEG
Star Wars d6 1e are all simultaneous declarations. MG, BW, and BE explicitly are write down your 3 actions for the turn. SW suggests writing them as a means of preventing «me too» declarations.
BW, BE, and MG all have essentially simultaneous 1st, simultaneous 2nd, and simultaneous 3rd.
WEG 1e has simultaneous declaration, but the action's skill roll total sets the specific sequence for the action subturn - All first actions in descending skill roll, all second actions in descending skill roll, all third actions in descending skill roll, etc...
Tunnels and Trolls is very lax, but within each step, it's all resolved simultaneously.
So, in the spell phase, anyone successfully casting generates the results, and they apply all at once.
Likewise saving rolls for stunts/stupid-PC-tricks.
Then, finally, the combat dice step. This step cannot be resolved other than simultaneously... since the participants in a particular knot of melee all total by side, then the difference between side totals is applied to the losing side...