Great question: relates to timing of reactions to Warlord commitments I think:
Had something similar at GamerzNexus the other day, and I think the answers will be related so will add it to this thread:
Was playing Ragnar vs Ragnar. We both had Blackmane Sentinels at other planets. Me at Planet 3, him at Planet 3. I also had a Ratling Sniper at 3, so I had command at 1+3, he at 2+4.
We then both committed our warlords, me to Planet 1, him to Planet 2. I had initiative.
Nature of the command was that if I moved my Sentinel to Planet 1 he'd aid my combat to some small degree, but unless he moved his Sentinel as well I'd lose my command win at Planet 3 as well, as it'd now be a draw. Thus I was only planning to move him if he did.
As I had initiative, he said I had to decide whether to move my Warlord or not (p13 rrg: Player with initiative has first opportunity to react). I said actually, if I passed then he passed, we'd stay put, but if I passed then he reacted, I'd have another chance to react again, as two consecutive passes are needed to close reaction window.
We read this and agreed, but then he went:
"But actually, we're not reacting to the same commit. We're reacting to two different commits. You have initiative, so your Warlord moves first."
Stupidly, I accepted his rightness in this.
Afterwards I checked the RRG p12 and noted that on Priority of Simultaneous Resolution, the player with initiative decides which effect resolves first. I could therefore have said that his commitment happens "simultaneously, but first".
In my scenario, Initiative was what mattered, as this determined which effect happened first (and thus which reaction window opened and closed first).
I now believe that I could have had the reaction window to his commitment complete in full before initiating the reaction window to my commit.
Is that right PB?
On a related note, I believe that the OP is even simpler, as they're to the same reaction window:
Shadowsun commits. Reaction window to this opens.
Shadowsun's player reacts with Shadowsun. That reaction completes in full, Blacksun Filter goes into play.
The opponent passes or reacts.
Shadowsun's player then reacts with Blacksun. That reaction completes in full.
Then both pass.
Then the reaction window closes.
Its my understanding that triggered abilities check their eligibility at the time they activate, not at the time the window opens. (vide foresight vs blackmane's hunt rules thread).
In your scenario, Initiative only determines who has first opportunity to react.
In the scenario where the opponent has a relevant reaction to his own Commit (like Blackmane's Hunt), Initiative becomes important, as in my example, but in determining which effect comes first