How do you run a pvp conflict

Up to now all my player's conflicts have been with NPCs. Suddenly I found myself in the middle of a face-off between two player-characters - a skinner and a brainer - but they didn't fight directly: they used an npc as a prop. Roughly, it went like this:

S: Hey Leroy, you've been brain-fucked by that girl. She's inside your mind - persuasion (roll=success)

... Leroy turns on B

B: Hey lover, you don't believe her, do you? In-brain puppet... Now come with me.
(roll=success)

... Leroy follows B

S: Hey Leroy - the town's burning around you - your duty is to the town.
persuasion (roll=success)

... Leroy steps away again.

... and this went on for a little longer in the same vein. No-one rolled less than 10 - and presumably if they carried on rolling well they could have played Leroy ping-pong all night.

I realise I could have interrupted with an NPC, or brought in another PC, but that feels like a copout - like I should really only be making a move if they fail. And I don't want to do damage to Leroy while they're rolling well. In fact, I want to give each character what they want.

But... how do you stop it getting ridiculous? They weren't willing to confront each other, and they both had skill enough to ensure nothing moved forward.

One thing you should do is, whoever acted first, have the other roll to interfere instead of waiting to react. Like this:

S: Hey Leroy, you've been brain-fucked by that girl. She's inside your mind.
B: Hey lover, you don't believe her, do you?
S rolls to persuade, B rolls to interfere.

That'll simplify many circumstances in play.

But your particular situation has a couple of really cool things in it that you could have done as well.

Thing 1: in-brain puppet strings doesn't let B directly control Leroy's actions the way S's persuasion does. Was the in-brain puppet strings a bluff? Have Leroy go along with S's persuasion, and ask B if he's going to trigger the in-brain puppet strings or what.

Thing 2: how does Leroy feel about this? When does he grow a spine? What's his threat type? You don't have to wait for someone to miss a roll to make a move. A standoff between people who keep hitting rolls is a perfect occasion for a move, especially a relatively soft move like the threat-appropriate version of "Leroy grows a spine."

If it was a case of Skinner Hypnotic holds vs. Brainer holds, you could play it out with seize by force, with the thing being Leroy's will, and the damage being done to their holds, their relationships to him, or even poor Leroy himself.  The two women snarling conflicting commands over his spasming, eye-bleeding body seems suitably Apocalyptic.

One thing you should do is, whoever acted first, have the other roll to interfere instead of waiting to react. Like this:

S: Hey Leroy, you've been brain-fucked by that girl. She's inside your mind.
B: Hey lover, you don't believe her, do you?
S rolls to persuade, B rolls to interfere.

That'll simplify many circumstances in play.

I was hoping to do this, but in each case the player waited to see what would happen before making their move - so it ended up like:

S: Hey Leroy, you've been brain-fucked by that girl. She's inside your mind.
B: <says nothing, looks at me and waits>
Me: Leroy hears what S says, and his eyes seem to clear. He looks at you with rage, and walks over, hands raised to strike.
B: Hey lover, you don't believe her, do you? <with some roleplay, too - this is a summary>
S: <says nothing, looks at me and waits>
Me: Leroy lowers his hands. His face goes slack for a moment, then he smiles at B.
S: Aren't you going to save the town
B: <says nothing, looks at me and waits>
... etc.

I did have Leroy start to do stuff, but each time the other player waited to see what would happen, then chose to interfere. That's also my difficulty with the history/aid rules - neither player wanted to do anything till they saw how the other roll resolved. They didn't actually want to go head to head.

I couldn't see how to progress, so the incident ended with Leroy back-handing the brainer - I guess he did grow a spine in the end ;)

Do you think, perhaps, that my difficulty was in waiting for input? I.e. after the first control should I have had Leroy do something irrevocable? As it was, I made him do something half-way (walk over with fist raised), then looked at the player to see if they were going to do anything about it. Perhaps that way of running it ends up ping-pong?

Call for an interfere roll before the dice hit the table. So, it's like this:

S: Hey Leroy, you've been brain-fucked by that girl. She's inside your mind.
Me <before dice are rolled>: Hold on, S. B, do you want to interfere? If so, how?
B: Sure.
or,
B: Nah, I'll see what happens.

Weaselheart: yeah! If B declines to interfere up-front, absolutely yes have Leroy do something irrevocable.

Also do look again and closely at how in-brain puppet strings works. The skinner and the brainer aren't equally matched so that they can play ping pong with someone. The skinner's playing golf and the brainer has a landmine.

Call for an interfere roll before the dice hit the table.

Weaselheart: yeah! If B declines to interfere up-front, absolutely yes have Leroy do something irrevocable.

... and a lightbulb comes on above my head! Thanks :)