''At this stage the pair of them will be in the squad for the Munster game,'' said assistant coach Sean Holley. ''We have to get Mike and the other World Cup boys involved, while making sure we maintain an element of continuity from the previous games. 'We don't anticipate any problems over selection, because it's not as if we have two 25-year-olds to choose from.
''They are at the opposite ends of the career spectrum. Mike has the chance to learn from a player who has been one of the best scrum-halves in world rugby, while Justin will be kept on his toes by having a younger rival to compete with.''
For all that, the pair are fiercely competitive, with Phillips driven on by an urge to prove himself the best No. 9 in Wales and Marshall's desire to be the best seemingly written into his DNA. It means the Ospreys have two potentially outstanding scrum-half assets as they look to put their season into gear.
''There are enough games to keep people happy,'' said Holley.
''We are clear that we want Mike to be the number one scrum-half for Wales, but we are also conscious of what Justin can offer and the invaluable role he can play at the region.
''Selection will be based on horses for courses and form, as it is for every position.
''Training has been great so far. It's been competitive, but there's plenty of mutual respect and I'm sure people will support each other as the campaign unfolds.''
Ian Gough and Ian Evans are likely to feature against Munster after figuring at Murrayfield, while Shane Williams and James Hook could also make the squad and maybe Nikki Walker after Scotland's World Cup elimination.
Meanwhile, Munster coach Declan Kidney includes Brian Carney and Peter Stringer in his extended squad announced yesterday. They join Alan Quinlan and Frank Sheahan from the RWC squad, both of whom featured in last week's game against Cardiff Blues.
Kidney is expected to name his starting XV tomorrow (Friday) and the squad fly out on Saturday from Cork Airport.