i think it must be presented as a very clear situation. only when there is NO doubt that these pirates are working as a team, they should be allowed to stage such an ambush. - and i mean it must be clear from the point of view of the ambushed player, not the pirates.
that involves indeed system chat - or - and thats better ... in local chat ( but its disabled atm ) - of course, adding a ship bigger than a gunboat ( even if its not interfering ) should not be allowed. - ignoring it is metagaming - and the point of not having a cruiser or bigger is that its too expensive to have these ships deployed for pirating ( at least for the simple act of "daily" piracy routine ).
to be safe and leave little room to misinterprete the situation, i d suggest to have both members of the group in scanner range ( like one or two lane sections, i think one section is 5000 meters? ) - then it should be clear that both pirates are one unit.
asking for a tax, but not pursuing the trader when he attempts to escape - but then opening fire instantly when he leaves a lane can and will cause misinterpretation of the situation, - especially when the second pirate will "finish the job, no matter what".