Here, here – straight from the Centaur’s mouth:
As stated on the CLERICS AFFECTING UNDEAD TABLE, this function may be attempted only once by each cleric. Of course, if there are two, both may attempt the function, each trying once, etc. There is also an exceptional case where turning may be practiced more than once by each and every cleric concerned. This occurs in cases where multiple forms of creatures subject to turning are involved. If the cleric attempting the turning is
successful against any or all types within the group of multiple forms of undead, that type or multiple types, to the maximum number indicated by the dice roll or otherwise indicated by the rules, are turned, and on the
next round the cleric so successful may attempt to turn other undead of the group. This process may continue as long as each successive attempt is successful and the cleric lives. Undead so turned (from the group of multiple types) are lowest hit dice types to highest hit dice types, i.e. first
listed to last listed an the table. Any failure to turn undead disallows a further attempt by the same cleric.
Turning can occur at the same time as missile discharge, magical device attacks, and/or spell casting. It also is
subject to initiative determination. If the undead are in a mixed group – for example, 1 vampire, 3 ghosts,
and 8 ghouls – you may opt to disallow any turning or other effect if the most powerful member – in the example above, the vampire – is not affected by the cleric. Naturally, this rule applies only to groups of mixed undead where the lesser are following or serving the greater. Mindless
undead, skeletons and zombies, cannot be considered. Otherwise, the cleric will affect undead according to the die score, with the possibility of the lesser monsters being turned or otherwise affected, while greater ones are unaffected.
DMG p.65 -emphasis mine-
So by these rules, both Matt and Junior could have tried a second turning attempt turn against the ghasts since they were each successful against the ghouls in last lesson. I am thinking that a party would need to be aware that some of the undead were of different types to know if they could turn again- although in that light- if the party were not sure whether or not the group was of mixed undead they could attempt turning again at the cost of an action that round as they try to turn what they suspect to be another type of undead only to discover they had simply failed to initially turn them.
Long story short: In the case of a group of mixed undead, if the first turning attempt is successful against one type of undead, another turning attempt can be made the next round. This continues until failure to turn, all undead types having been turned, or death of the divine spell caster. However if the group of mixed undead is under thrall to a more powerful undead in the group – the strongest undead must be turned the first time or no other attempts may be made against that group by that player.